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Request For Comments - RFC4632

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Network Working Group                                          V. Fuller
Request for Comments: 4632                                 Cisco Systems
BCP: 122                                                           T. Li
Obsoletes: 1519                                          Tropos Networks
Category: Best Current Practice                              August 2006

                Classless Inter-domain Routing (CIDR):
          The Internet Address Assignment and Aggregation Plan

Status of This Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).


   This memo discusses the strategy for address assignment of the
   existing 32-bit IPv4 address space with a view toward conserving the
   address space and limiting the growth rate of global routing state.
   This document obsoletes the original Classless Inter-domain Routing
   (CIDR) spec in RFC 1519, with changes made both to clarify the
   concepts it introduced and, after more than twelve years, to update
   the Internet community on the results of deploying the technology

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RFC 4632                 CIDR Address Strategy               August 2006

Table of Contents

   1. Introduction ....................................................3
   2. History and Problem Description .................................3
   3. Classless Addressing as a Solution ..............................4
      3.1. Basic Concept and Prefix Notation ..........................5
   4. Address Assignment and Routing Aggregation ......................8
      4.1. Aggregation Efficiency and Limitations .....................8
      4.2. Distributed Assignment of Address Space ...................10
   5. Routing Implementation Considerations ..........................11
      5.1. Rules for Route Advertisement .............................11
      5.2. How the Rules Work ........................................12
      5.3. A Note on Prefix Filter Formats ...........................13
      5.4. Responsibility for and Configuration of Aggregation .......13
      5.5. Route Propagation and Routing Protocol Considerations .....15
   6. Example of New Address Assignments and Routing .................15
      6.1. Address Delegation ........................................15
      6.2. Routing Advertisements ....................................17
   7. Domain Name Service Considerations .............................18
   8. Transition to a Long-Term Solution .............................18
   9. Analysis of CIDR's Effect on Global Routing State ..............19
   10. Conclusions and Recommendations ...............................20
   11. Status Updates to CIDR Documents ..............................21
   12. Security Considerations .......................................23
   13. Acknowledgements ..............................................24
   14. References ....................................................25
      14.1. Normative References .....................................25
      14.2. Informative References ...................................25

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RFC 4632                 CIDR Address Strategy               August 2006

1.  Introduction

   This memo discusses the strategy for address assignment of the
   existing 32-bit IPv4 address space with a view toward conserving the
   address space and limiting the growth rate of global routing state.
   This document obsoletes the original CIDR spec [RFC1519], with
   changes made both to clarify the concepts it introduced and, after
   more than twelve years, to update the Internet community on the
   results of deploying the technology described.

2.  History and Problem Description

   What is now known as the Internet started as a research project in
   the 1970s to design and develop a set of protocols that could be used
   with many different network technologies to provide a seamless, end-
   to-end facility for interconnecting a diverse set of end systems.
   When it was determined how the 32-bit address space would be used,
   certain assumptions were made about the number of organizations to be
   connected, the number of end systems per organization, and total
   number of end systems on the network.  The end result was the
   establishment (see [RFC791]) of three classes of networks: Class A
   (most significant address bits '00'), with 128 possible networks each
   and 16777216 end systems (minus special bit values reserved for
   network/broadcast addresses); Class B (MSB '10'), with 16384 possible
   networks each with 65536 end systems (less reserved values); and
   Class C (MSB '110'), and 2097152 possible networks each and 254 end
   systems (256 bit combinations minus the reserved all-zeros and all-
   ones patterns).  The set of addresses with MSB '111' was reserved for
   future use; parts of this were eventually defined (MSB '1110') for
   use with IPv4 multicast and parts are still reserved as of the
   writing of this document.

   In the late 1980s, the expansion and commercialization of the former
   research network resulted in the connection of many new organizations
   to the rapidly growing Internet, and each new organization required
   an address assignment according to the Class A/B/C addressing plan.
   As demand for new network numbers (particularly in the Class B space)
   took what appeared to be an exponential growth rate, some members of
   the operations and engineering community started to have concerns
   over the long-term scaling properties of the class A/B/C system and
   began thinking about how to modify network number assignment policy
   and routing protocols to accommodate the growth.  In November, 1991,
   the Internet Engineering Task Force (IETF) created the ROAD (Routing
   and Addressing) group to examine the situation.  This group met in
   January 1992 and identified three major problems:

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RFC 4632                 CIDR Address Strategy               August 2006

   1.  Exhaustion of the Class B network address space.  One fundamental
       cause of this problem is the lack of a network class of a size
       that is appropriate for mid-sized organization.  Class C, with a
       maximum of 254 host addresses, is too small, whereas Class B,
       which allows up to 65534 host addresses, is too large for most
       organizations but was the best fit available for use with

   2.  Growth of routing tables in Internet routers beyond the ability
       of current software, hardware, and people to effectively manage.

   3.  Eventual exhaustion of the 32-bit IPv4 address space.

       It was clear that then-current rates of Internet growth would
       cause the first two problems to become critical sometime between
       1993 and 1995.  Work already in progress on topological
       assignment of addressing for Connectionless Network Service
       (CLNS), which was presented to the community at the Boulder IETF
       in December of 1990, led to thoughts on how to re-structure the
       32-bit IPv4 address space to increase its lifespan.  Work in the
       ROAD group followed and eventually resulted in the publication of
       [RFC1338], and later, [RFC1519].

       The design and deployment of CIDR was intended to solve these
       problems by providing a mechanism to slow the growth of global
       routing tables and to reduce the rate of consumption of IPv4
       address space.  It did not and does not attempt to solve the
       third problem, which is of a more long-term nature; instead, it
       endeavors to ease enough of the short- to mid-term difficulties
       to allow the Internet to continue to function efficiently while
       progress is made on a longer-term solution.

       More historical background on this effort and on the ROAD group
       may be found in [RFC1380] and at [LWRD].

3.  Classless Addressing as a Solution

   The solution that the community created was to deprecate the Class
   A/B/C network address assignment system in favor of using
   "classless", hierarchical blocks of IP addresses (referred to as
   prefixes).  The assignment of prefixes is intended to roughly follow
   the underlying Internet topology so that aggregation can be used to
   facilitate scaling of the global routing system.  One implication of
   this strategy is that prefix assignment and aggregation is generally
   done according to provider-subscriber relationships, since that is
   how the Internet topology is determined.

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   When originally proposed in [RFC1338] and [RFC1519], this addressing
   plan was intended to be a relatively short-term response, lasting
   approximately three to five years, during which a more permanent
   addressing and routing architecture would be designed and
   implemented.  As can be inferred from the dates on the original
   documents, CIDR has far outlasted its anticipated lifespan and has
   become the mid-term solution to the problems described above.

   Note that in the following text we describe the current policies and
   procedures that have been put in place to implement the allocation
   architecture discussed here.  This description is not intended to be
   interpreted as direction to IANA.

   Coupled with address management strategies implemented by the
   Regional Internet Registries (see [NRO] for details), the deployment
   of CIDR-style addressing has also reduced the rate at which IPv4
   address space has been consumed, thus providing short- to medium-term
   relief to problem #3, described above.

   Note that, as defined, this plan neither requires nor assumes the
   re-assignment of those parts of the legacy "Class C" space that are
   not amenable to aggregation (sometimes called "the swamp").  Doing so
   would somewhat reduce routing table sizes (current estimate is that
   "the swamp" contains approximately 15,000 entries), though at a
   significant renumbering cost.  Similarly, there is no hard
   requirement that any end site renumber when changing transit service
   provider, but end sites are encouraged do so to eliminate the need
   for explicit advertisement of their prefixes into the global routing

3.1.  Basic Concept and Prefix Notation

   In the simplest sense, the change from Class A/B/C network numbers to
   classless prefixes is to make explicit which bits in a 32-bit IPv4
   address are interpreted as the network number (or prefix) associated
   with a site and which are the used to number individual end systems
   within the site.  In CIDR notation, a prefix is shown as a 4-octet
   quantity, just like a traditional IPv4 address or network number,
   followed by the "/" (slash) character, followed by a decimal value
   between 0 and 32 that describes the number of significant bits.

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   For example, the legacy "Class B" network, with an implied
   network mask of, is defined as the prefix,
   the "/16" indicating that the mask to extract the network portion of
   the prefix is a 32-bit value where the most significant 16 bits are
   ones and the least significant 16 bits are zeros.  Similarly, the
   legacy "Class C" network number is defined as the prefix; the most significant 24 bits are ones and the least
   significant 8 bits are zeros.

   Using classless prefixes with explicit prefix lengths allows much
   more flexible matching of address space blocks according to actual
   need.  Where formerly only three network sizes were available,
   prefixes may be defined to describe any power of two-sized block of
   between one and 2^32 end system addresses.  In practice, the
   unallocated pool of addresses is administered by the Internet
   Assigned Numbers Authority ([IANA]).  The IANA makes allocations from
   this pool to Regional Internet Registries, as required.  These
   allocations are made in contiguous bit-aligned blocks of 2^24
   addresses (a.k.a. /8 prefixes).  The Regional Internet Registries
   (RIRs), in turn, allocate or assign smaller address blocks to Local
   Internet Registries (LIRs) or Internet Service Providers (ISPs).
   These entities may make direct use of the assignment (as would
   commonly be the case for an ISP) or may make further sub-allocations
   of addresses to their customers.  These RIR address assignments vary
   according to the needs of each ISP or LIR.  For example, a large ISP
   might be allocated an address block of 2^17 addresses (a /15 prefix),
   whereas a smaller ISP may be allocated an address block of 2^11
   addresses (a /21 prefix).

   Note that the terms "allocate" and "assign" have specific meaning in
   the Internet address registry system; "allocate" refers to the
   delegation of a block of address space to an organization that is
   expected to perform further sub-delegations, and "assign" is used for
   sites that directly use (i.e., number individual hosts) the block of
   addresses received.

   The following table provides a convenient shortcut to all the CIDR
   prefix sizes, showing the number of addresses possible in each prefix
   and the number of prefixes of that size that may be numbered in the
   32-bit IPv4 address space:

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       notation       addrs/block      # blocks
       --------       -----------     ----------
       n.n.n.n/32               1     4294967296    "host route"
       n.n.n.x/31               2     2147483648    "p2p link"
       n.n.n.x/30               4     1073741824
       n.n.n.x/29               8      536870912
       n.n.n.x/28              16      268435456
       n.n.n.x/27              32      134217728
       n.n.n.x/26              64       67108864
       n.n.n.x/25             128       33554432
       n.n.n.0/24             256       16777216    legacy "Class C"
       n.n.x.0/23             512        8388608
       n.n.x.0/22            1024        4194304
       n.n.x.0/21            2048        2097152
       n.n.x.0/20            4096        1048576
       n.n.x.0/19            8192         524288
       n.n.x.0/18           16384         262144
       n.n.x.0/17           32768         131072
       n.n.0.0/16           65536          65536    legacy "Class B"
       n.x.0.0/15          131072          32768
       n.x.0.0/14          262144          16384
       n.x.0.0/13          524288           8192
       n.x.0.0/12         1048576           4096
       n.x.0.0/11         2097152           2048
       n.x.0.0/10         4194304           1024
       n.x.0.0/9          8388608            512
       n.0.0.0/8         16777216            256    legacy "Class A"
       x.0.0.0/7         33554432            128
       x.0.0.0/6         67108864             64
       x.0.0.0/5        134217728             32
       x.0.0.0/4        268435456             16
       x.0.0.0/3        536870912              8
       x.0.0.0/2       1073741824              4
       x.0.0.0/1       2147483648              2       4294967296              1    "default route"

   n is an 8-bit decimal octet value.  Point-to-point links are
   discussed in more detail in [RFC3021].

   x is a 1- to 7-bit value, based on the prefix length, shifted into
   the most significant bits of the octet and converted into decimal
   form; the least significant bits of the octet are zero.

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   In practice, prefixes of length shorter than 8 have not been
   allocated or assigned to date, although routes to such short prefixes
   may exist in routing tables if or when aggressive aggregation is
   performed.  As of the writing of this document, no such routes are
   seen in the global routing system, but operator error and other
   events have caused some of them (i.e., and
   to be observed in some networks at some times in the past.

4.  Address Assignment and Routing Aggregation

   Classless addressing and routing was initially developed primarily to
   improve the scaling properties of routing on the global Internet.
   Because the scaling of routing is very tightly coupled to the way
   that addresses are used, deployment of CIDR had implications for the
   way in which addresses were assigned.

4.1.  Aggregation Efficiency and Limitations

   The only commonly understood method for reducing routing state on a
   packet-switched network is through aggregation of information.  For
   CIDR to succeed in reducing the size and growth rate of the global
   routing system, the IPv4 address assignment process needed to be
   changed to make possible the aggregation of routing information along
   topological lines.  Since, in general, the topology of the network is
   determined by the service providers who have built it, topologically
   significant address assignments are necessarily service-provider

   Aggregation is simple for an end site that is connected to one
   service provider: it uses address space assigned by its service
   provider, and that address space is a small piece of a larger block
   allocated to the service provider.  No explicit route is needed for
   the end site; the service provider advertises a single aggregate
   route for the larger block.  This advertisement provides reachability
   and routeability for all the customers numbered in the block.

   There are two, more complex, situations that reduce the effectiveness
   of aggregation:

   o  An organization that is multi-homed.  Because a multi-homed
      organization must be advertised into the system by each of its
      service providers, it is often not feasible to aggregate its
      routing information into the address space of any one of those
      providers.  Note that the organization still may receive its
      address assignment out of a service provider's address space
      (which has other advantages), but that a route to the
      organization's prefix is, in the most general case, explicitly
      advertised by all of its service providers.  For this reason, the

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      global routing cost for a multi-homed organization is generally
      the same as it was prior to the adoption of CIDR.  A more detailed
      consideration of multi-homing practices can be found in [RFC4116].

   o  An organization that changes service provider but does not
      renumber.  This has the effect of "punching a hole" in one of the
      original service provider's aggregated route advertisements.  CIDR
      handles this situation by requiring that the newer service
      provider to advertise a specific advertisement for the re-homed
      organization; this advertisement is preferred over provider
      aggregates because it is a longer match.  To maintain efficiency
      of aggregation, it is recommended that an organization that
      changes service providers plan eventually to migrate its network
      into a an prefix assigned from its new provider's address space.
      To this end, it is recommended that mechanisms to facilitate such
      migration, such as dynamic host address assignment that uses
      [RFC2131]), be deployed wherever possible, and that additional
      protocol work be done to develop improved technology for

   Note that some aggregation efficiency gain can still be had for
   multi-homed sites (and, in general, for any site composed of
   multiple, logical IPv4 networks); by allocating a contiguous power-
   of-two block address space to the site (as opposed to multiple,
   independent prefixes), the site's routing information may be
   aggregated into a single prefix.  Also, since the routing cost
   associated with assigning a multi-homed site out of a service
   provider's address space is no greater than the old method of
   sequential number assignment by a central authority, it makes sense
   to assign all end-site address space out of blocks allocated to
   service providers.

   It is also worthwhile to mention that since aggregation may occur at
   multiple levels in the system, it may still be possible to aggregate
   these anomalous routes at higher levels of whatever hierarchy may be
   present.  For example, if a site is multi-homed to two relatively
   small providers that both obtain connectivity and address space from
   the same large provider, then aggregation by the large provider of
   routes from the smaller networks will include all routes to the
   multi-homed site.  The feasibility of this sort of second-level
   aggregation depends on whether topological hierarchy exists among a
   site, its directly-connected providers, and other providers to which
   they are connected; it may be practical in some regions of the global
   Internet but not in others.

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   Note: In the discussion and examples that follow, prefix notation is
   used to represent routing destinations.  This is used for
   illustration only and does not require that routing protocols use
   this representation in their updates.

4.2.  Distributed Assignment of Address Space

   In the early days of the Internet, IPv4 address space assignment was
   performed by the central Network Information Center (NIC).  Class
   A/B/C network numbers were assigned in essentially arbitrary order,
   roughly according to the size of the organizations that requested
   them.  All assignments were recorded centrally, and no attempt was
   made to assign network numbers in a manner that would allow routing

   When CIDR was originally deployed, the central assignment authority
   continued to exist but changed its procedures to assign large blocks
   of "Class C" network numbers to each service provider.  Each service
   provider, in turn, assigned bitmask-oriented subsets of the
   provider's address space to each customer.  This worked reasonably
   well, as long as the number of service providers was relatively small
   and relatively constant, but it did not scale well, as the number of
   service providers grew at a rapid rate.

   As the Internet started to expand rapidly in the 1990s, it became
   clear that a single, centralized address assignment authority was
   problematic.  This function began being de-centralized when address
   space assignment for European Internet sites was delegated in bit-
   aligned blocks of 16777216 addresses (what CIDR would later define as
   a /8) to the RIPE NCC ([RIPE]), effectively making it the first of
   the RIRs.  Since then, address assignment has been formally
   distributed as a hierarchical function with IANA, the RIRs, and the
   service providers.  Removing the bottleneck of a single organization
   having responsibility for the global Internet address space greatly
   improved the efficiency and response time for new assignments.

   Hierarchical delegation of addresses in this manner implies that
   sites with addresses assigned out of a given service provider are,
   for routing purposes, part of that service provider and will be
   routed via its infrastructure.  This implies that routing information
   about multi-homed organizations (i.e., organizations connected to
   more than one network service provider) will still need to be known
   by higher levels in the hierarchy.

   A historical perspective on these issues is described in [RFC1518].
   Additional discussion may also be found in [RFC3221].

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5.  Routing Implementation Considerations

   With the change from classful network numbers to classless prefixes,
   it is not possible to infer the network mask from the initial bit
   pattern of an IPv4 address.  This has implications for how routing
   information is stored and propagated.  Network masks or prefix
   lengths must be explicitly carried in routing protocols.  Interior
   routing protocols, such as OSPF [RFC2328], Intermediate System to
   Intermediate System (IS-IS) [RFC1195], RIPv2 [RFC2453], and Cisco
   Enhanced Interior Gateway Routing Protocol (EIGRP), and the BGP4
   exterior routing protocol [RFC4271], all support this functionality,
   having been developed or modified as part of the deployment of
   classless inter-domain routing during the 1990s.

   Older interior routing protocols, such as RIP [RFC1058], HELLO, and
   Cisco Interior Gateway Routing Protocol (IGRP), and older exterior
   routing protocols, such as Exterior Gateway Protocol (EGP) [RFC904],
   do not support explicit carriage of prefix length/mask and thus
   cannot be effectively used on the Internet other than in very limited
   stub configurations.  Although their use may be appropriate in simple
   legacy end-site configurations, they are considered obsolete and
   should NOT be used in transit networks connected to the global

   Similarly, routing and forwarding tables in layer-3 network equipment
   must be organized to store both prefix and prefix length or mask.
   Equipment that organizes its routing/forwarding information according
   to legacy Class A/B/C network/subnet conventions cannot be expected
   to work correctly on networks connected to the global Internet; use
   of such equipment is not recommended.  Fortunately, very little such
   equipment is in use today.

5.1.  Rules for Route Advertisement

   1.  Forwarding in the Internet is done on a longest-match basis.
       This implies that destinations that are multi-homed relative to a
       routing domain must always be explicitly announced into that
       routing domain (i.e., they cannot be summarized).  If a network
       is multi-homed, all of its paths into a routing domain that is
       "higher" in the hierarchy of networks must be known to the
       "higher" network).

   2.  A router that generates an aggregate route for multiple, more-
       specific routes must discard packets that match the aggregate
       route, but not any of the more-specific routes.  In other words,
       the "next hop" for the aggregate route should be the null
       destination.  This is necessary to prevent forwarding loops when
       some addresses covered by the aggregate are not reachable.

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   Note that during failures, partial routing of traffic to a site that
   takes its address space from one service provider but that is
   actually reachable only through another (i.e., the case of a site
   that has changed service providers) may occur because such traffic
   will be forwarded along the path advertised by the aggregated route.
   Rule #2 will prevent packet misdelivery by causing such traffic to be
   discarded by the advertiser of the aggregated route, but the output
   of "traceroute" and other similar tools will suggest that a problem
   exists within that network rather than in the network that is no
   longer advertising the more-specific prefix.  This may be confusing
   to those trying to diagnose connectivity problems; see the example in
   Section 6.2 for details.  A solution to this perceived "problem" is
   beyond the scope of this document; it lies with better education of
   the user/operator community, not in routing technology.

   An implementation following these rules should also be generalized,
   so that an arbitrary network number and mask are accepted for all
   routing destinations.  The only outstanding constraint is that the
   mask must be left contiguous.  Note that the degenerate route to
   prefix is used as a default route and MUST be accepted by
   all implementations.  Further, to protect against accidental
   advertisements of this route via the inter-domain protocol, this
   route should only be advertised to another routing domain when a
   router is explicitly configured to do so, never as a non-configured,
   "default" option.

5.2.  How the Rules Work

   Rule #1 guarantees that the forwarding algorithm used is consistent
   across routing protocols and implementations.  Multi-homed networks
   are always explicitly advertised by every service provider through
   which they are routed, even if they are a specific subset of one
   service provider's aggregate (if they are not, they clearly must be
   explicitly advertised).  It may seem as if the "primary" service
   provider could advertise the multi-homed site implicitly as part of
   its aggregate, but longest-match forwarding causes this not to work.
   More details are provided in [RFC4116].

   Rule #2 guarantees that no routing loops form due to aggregation.
   Consider a site that has been assigned 192.168.64/19 by its "parent"
   provider, which has  The "parent" network will
   advertise to the "child" network.  If the "child"
   network were to lose internal connectivity to (which
   is part of its aggregate), traffic from the "parent" to the to the
   "child" destined for will follow the "child's"
   advertised route.  When that traffic gets to the "child", however,
   the child *must not* follow the route back up to the
   "parent", since that would result in a forwarding loop.  Rule #2 says

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   that the "child" may not follow a less-specific route for a
   destination that matches one of its own aggregated routes (typically,
   this is implemented by installing a "discard" or "null" route for all
   aggregated prefixes that one network advertises to another).  Note
   that handling of the "default" route ( is a special case of
   this rule; a network must not follow the default to destinations that
   are part of one of its aggregated advertisements.

5.3.  A Note on Prefix Filter Formats

   Systems that process route announcements must be able to verify that
   information that they receive is acceptable according to policy
   rules.  Implementations that filter route advertisements must allow
   masks or prefix lengths in filter elements.  Thus, filter elements
   that formerly were specified as


   now look something like this:


   This is merely making explicit the network mask that was implied by
   the Class A/B/C classification of network numbers.  It is also useful
   to enhance filtering capability to allow the match of a prefix and
   all more-specific prefixes with the same bit pattern; fortunately,
   this functionality has been implemented by most vendors of equipment
   used on the Internet.

5.4.  Responsibility for and Configuration of Aggregation

   Under normal circumstances, a routing domain (or "Autonomous System")
   that has been allocated or assigned a set of prefixes has sole
   responsibility for aggregation of those prefixes.  In the usual case,
   the AS will install configuration in one or more of its routers to
   generate aggregate routes based on more-specific routes known to its
   internal routing system.  These aggregate routes are advertised into
   the global routing system by the border routers for the routing
   domain.  The more-specific internal routes that overlap with the
   aggregate routes should not be advertised globally.  In some cases,

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   an AS may wish to delegate aggregation responsibility to another AS
   (for example, a customer may wish for its service provider to
   generate aggregated routing information on its behalf); in such
   cases, aggregation is performed by a router in the second AS
   according to the routes that it receives from the first, combined
   with configured policy information describing how those routes should
   be aggregated.

   Note that one provider may choose to perform aggregation on the
   routes it receives from another without explicit agreement; this is
   termed "proxy aggregation".  This can be a useful tool for reducing
   the amount of routing state that an AS must carry and propagate to
   its customers and neighbors.  However, proxy aggregation can also
   create unintended consequences in traffic engineering.  Consider what
   happens if both AS 2 and 3 receive routes from AS 1 but AS 2 performs
   proxy aggregation while AS 3 does not.  Other ASes that receive
   transit routing information from both AS 2 and AS 3 will see an
   inconsistent view of the routing information originated by AS 1.
   This may cause an unexpected shift of traffic toward AS 1 through AS
   3 for AS 3's customers and any others receiving transit routes from
   AS 3.  Because proxy aggregation can cause unanticipated consequences
   for parts of the Internet that have no relationship with either the
   source of the aggregated routes or the party providing aggregation,
   it should be used with extreme caution.

   Configuration of the routes to be combined into aggregates is an
   implementation of routing policy and requires some manually
   maintained information.  As an addition to the information that must
   be maintained for a set of routeable prefixes, aggregation
   configuration is typically just a line or two defining the range of
   the block of IPv4 addresses to be aggregated.  A site performing its
   own aggregation is doing so for address blocks that it has been
   assigned; a site performing aggregation on behalf of another knows
   this information because of an agreement to delegate aggregation.
   Assuming that the best common practice for network administrators is
   to exchange lists of prefixes to accept from each other,
   configuration of aggregation information does not introduce
   significant additional administrative overhead.

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   The generation of an aggregate route is usually specified either
   statically or in response to learning an active dynamic route for a
   prefix contained within the aggregate route.  If such dynamic
   aggregate route advertisement is done, care should be taken that
   routes are not excessively added or withdrawn (known as "route
   flapping").  In general, a dynamic aggregate route advertisement is
   added when at least one component of the aggregate becomes reachable
   and it is withdrawn only when all components become unreachable.
   Properly configured, aggregated routes are more stable than non-
   aggregated routes and thus improve global routing stability.

   Implementation note: Aggregation of the "Class D" (multicast) address
   space is beyond the scope of this document.

5.5.  Route Propagation and Routing Protocol Considerations

   Prior to the original deployment of CIDR, common practice was to
   propagate routes learned via exterior routing protocols (i.e., EGP or
   BGP) through a site's interior routing protocol (typically, OSPF,
   IS-IS, or RIP).  This was done to ensure that consistent and correct
   exit points were chosen for traffic to be sent to a destination
   learned through those protocols.  Four evolutionary effects -- the
   advent of CIDR, explosive growth of global routing state, widespread
   adoption of BGP4, and a requirement to propagate full path
   information -- have combined to deprecate that practice.  To ensure
   proper path propagation and prevent inter-AS routing inconsistency
   (BGP4's loop detection/prevention mechanism requires full path
   propagation), transit networks must use internal BGP (iBGP) for
   carrying routes learned from other providers both within and through
   their networks.

6.  Example of New Address Assignments and Routing

6.1.  Address Delegation

   Consider the block of 524288 (2^19) addresses, beginning with and ending with, allocated to a single
   network provider, "PA".  This is equivalent in size to a block of
   2048 legacy "Class C" network numbers (or /24s).  A classless route
   to this block would be described as with a mask of and the prefix

   Assume that this service provider connects six sites in the following
   order (significant because it demonstrates how temporary "holes" may
   form in the service provider's address space):

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   o  "C1", requiring fewer than 2048 addresses (/21 or 8 x /24)

   o  "C2", requiring fewer than 4096 addresses (/20 or 16 x /24)

   o  "C3", requiring fewer than 1024 addresses (/22 or 4 x /24)

   o  "C4", requiring fewer than 1024 addresses (/22 or 4 x /24)

   o  "C5", requiring fewer than 512 addresses (/23 or 2 x /24)

   o  "C6", requiring fewer than 512 addresses (/23 or 2 x /24)

   In all cases, the number of IPv4 addresses "required" by each site is
   assumed to allow for significant growth.  The service provider
   delegates its address space as follows:

   o  C1.  assign 10.24.0 through 10.24.7.  This block of networks is
      described by the route (mask

   o  C2.  Assign 10.24.16 through 10.24.31.  This block is described by
      the route (mask

   o  C3.  Assign 10.24.8 through 10.24.11.  This block is described by
      the route (mask

   o  C4.  Assign 10.24.12 through 10.24.15.  This block is described by
      the route (mask

   o  C5.  Assign 10.24.32 and 10.24.33.  This block is described by the
      route (mask

   o  C6.  Assign 10.24.34 and 10.24.35.  This block is described by the
      route (mask

   These six sites should be represented as six prefixes of varying size
   within the provider's IGP.  If, for some reason, the provider uses an
   obsolete IGP that doesn't support classless routing or variable-
   length subnets, then explicit routes for all /24s will have to be

   To make this example more realistic, assume that C4 and C5 are multi-
   homed through some other service provider, "PB".  Further assume the
   existence of a site, "C7", that was originally connected to "RB" but
   that has moved to "PA".  For this reason, it has a block of network
   numbers that are assigned out PB's block of (the next) 2048 x /24.

   o  C7.  Assign 10.32.0 through 10.32.15.  This block is described by
      the route (mask

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   For the multi-homed sites, assume that C4 is advertised as primary
   via "RA" and secondary via "RB"; and that C5 is primary via "RB" and
   secondary via "RA".  In addition, assume that "RA" and "RB" are both
   connected to the same transit service provider, "BB".

   Graphically, this topology looks something like this: --         __10.32.0.0 -
   C1:        \       /  C7:
                            \     /
                             +----+                              +----+ -  |    |                              |    |
   C2:       \ |    |  _10.24.12.0 -  |    |
                            \|    | / C4:        \ |    |
                             |    |/                            \|    | -| PA |\                             | PB |
   C3:          |    | \__10.24.32.0 -|    |
                             |    |    C5:         |    |
                             |    |                              |    | -|    |                              |    |
   C6:         |    |                              |    |
                             +----+                              +----+
                               ||                                  ||
   routing advertisements:     ||                                  ||
                               ||                                  ||
  (C4)  ||     (C4)  ||
  (C7)   ||     (C5)  ||
  (PA)   ||     (PB)   ||
                               ||                                  ||
                               VV                                  VV
                            +---------- BACKBONE NETWORK BB ----------+

6.2.  Routing Advertisements

   To follow rule #1, PA will need to advertise the block of addresses
   that it was given and C7.  Since C4 is multi-homed and primary
   through PA, it must also be advertised.  C5 is multi-homed and
   primary through PB.  In principle (and in the example above), it need
   not be advertised, since longest match by PB will automatically
   select PB as primary and the advertisement of PA's aggregate will be
   used as a secondary.  In actual practice, C5 will normally be
   advertised via both providers.

   Advertisements from "PA" to "BB" will be primary    (advertises C4) primary     (advertises C7) primary     (advertises remainder of PA)

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   For PB, the advertisements must also include C4 and C5, as well as
   its block of addresses.

   Advertisements from "PB" to "BB" will be secondary  (advertises C4) primary    (advertises C5) primary     (advertises remainder of RB)

   To illustrate the problem diagnosis issue mentioned in Section 5.1,
   consider what happens if PA loses connectivity to C7 (the site that
   is assigned out of PB's space).  In a stateful protocol, PA will
   announce to BB that has become unreachable.  Now, when
   BB flushes this information out of its routing table, any future
   traffic sent through it for this destination will be forwarded to PB
   (where it will be dropped according to Rule #2) by virtue of PB's
   less-specific match,  Although this does not cause an
   operational problem (C7 is unreachable in any case), it does create
   some extra traffic across "BB" (and may also prove confusing to
   someone trying to debug the outage with "traceroute").  A mechanism
   to cache such unreachable state might be nice, but it is beyond the
   scope of this document.

7.  Domain Name Service Considerations

   One aspect of Internet services that was notably affected by the move
   to CIDR was the mechanism used for address-to-name translation:  the
   IN-ADDR.ARPA zone of the domain system.  Because this zone is
   delegated on octet boundaries only, the move to an address assignment
   plan that uses bitmask-oriented addressing caused some increase in
   work for those who maintain parts of the IN-ADDR.ARPA zone.

   A description of techniques to populate the IN-ADDR.ARPA zone when
   and used address that blocks that do not align to octet boundaries is
   described in [RFC2317].

8.  Transition to a Long-Term Solution

   CIDR was designed to be a short-term solution to the problems of
   routing state and address depletion on the IPv4 Internet.  It does
   not change the fundamental Internet routing or addressing
   architectures.  It is not expected to affect any plans for transition
   to a more long-term solution except, perhaps, by delaying the urgency
   of developing such a solution.

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9.  Analysis of CIDR's Effect on Global Routing State

   When CIDR was first proposed in the early 1990s, the original authors
   made some observations about the growth rate of global routing state
   and offered projections on how CIDR deployment would, hopefully,
   reduce what appeared to be exponential growth to a more sustainable
   rate.  Since that deployment, an ongoing effort, called "The CIDR
   Report" [CRPT], has attempted to quantify and track that growth rate.
   What follows is a brief summary of the CIDR report as of March 2005,
   with an attempt to explain the various patterns and changes of growth
   rate that have occurred since measurements of the size of global
   routing state began in 1988.

   When the graph of "Active BGP Table Entries" [CBGP] is examined,
   there appear to be several different growth trends with distinct
   inflection points reflecting changes in policy and practice.  The
   trends and events that are believed to have caused them were as

   1.  Exponential growth at the far left of the graph.  This represents
       the period of early expansion and commercialization of the former
       research network, from the late 1980s through approximately 1994.
       The major driver for this growth was a lack of aggregation
       capability for transit providers, and the widespread use of
       legacy Class C allocations for end sites.  Each time a new site
       was connected to the global Internet, one or more new routing
       entries were generated.

   2.  Acceleration of the exponential trend in late 1993 and early 1994
       as CIDR "supernet" blocks were first assigned by the NIC and
       routed as separate legacy class-C networks by service provider.

   3.  A sharp drop in 1994 as BGP4 deployment by providers allowed
       aggregation of the "supernet" blocks.  Note that the periods of
       largest declines in the number of routing table entries typically
       correspond to the weeks following each meeting of the IETF CIDR
       Deployment Working Group.

   4.  Roughly linear growth from mid-1994 to early 1999 as CIDR-based
       address assignments were made and aggregated routes added
       throughout the network.

   5.  A new period of exponential growth again from early 1999 until
       2001 as the "high-tech bubble" fueled both rapid expansion of the
       Internet, as well as a large increase in more-specific route
       advertisements for multi-homing and traffic engineering.

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   6.  Flattening of growth through 2001 caused by a combination of the
       "dot-com bust", which caused many organizations to cease
       operations, and the "CIDR police" [CPOL] work aimed at improving
       aggregation efficiency.

   7.  Roughly linear growth through 2002 and 2003.  This most likely
       represents a resumption of the "normal" growth rate observed
       before the "bubble", as well as an end to the "CIDR Police"

   8.  A more recent trend of exponential growth beginning in 2004.  The
       best explanation would seem to be an improvement of the global
       economy driving increased expansion of the Internet and the
       continued absence of the "CIDR Police" effort, which previously
       served as an educational tool for new providers to improve
       aggregation efficiency.  There have also been some cases where
       service providers have deliberately de-aggregated prefixes in an
       attempt to mitigate security problems caused by conflicting route
       advertisements (see Section 12).  Although this behavior may
       solve the short-term problems seen by such providers, it is
       fundamentally non-scalable and quite detrimental to the community
       as a whole.  In addition, there appear to be many providers
       advertising both their allocated prefixes and all the /24
       components thereof, probably due to a lack of consistent current
       information about recommended routing configuration.

10.  Conclusions and Recommendations

   In 1992, when CIDR was first developed, there were serious problems
   facing the continued growth of the Internet.  Growth in routing state
   complexity and the rapid increase in consumption of address space
   made it appear that one or both problems would preclude continued
   growth of the Internet within a few short years.

   Deployment of CIDR, in combination with BGP4's support for carrying
   classless prefix routes, alleviated the short-term crisis.  It was
   only through a concerted effort by both the equipment manufacturers
   and the provider community that this was achieved.  The threat (and,
   perhaps in some cases, actual implementation of) charging networks
   for advertising prefixes may have offered an additional incentive to
   share the address space, and thus the associated costs of advertising
   routes to service providers.

   The IPv4 routing system architecture carries topology information
   based on aggregate address advertisements and a collection of more-
   specific advertisements that are associated with traffic engineering,
   multi-homing, and local configuration.  As of March 2005, the base
   aggregate address load in the routing system has some 75,000 entries.

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   Approximately 85,000 additional entries are more specific entries of
   this base "root" collection.  There is reason to believe that many of
   these additional entries exist to solve problems of regional or even
   local scope and should not need to be globally propagated.

   An obvious question to ask is whether CIDR can continue to be a
   viable approach to keeping global routing state growth and address
   space depletion at sustainable rates.  Recent measurements indicate
   that exponential growth has resumed, but further analysis suggests
   that this trend can be mitigated by a more active effort to educate
   service providers as to efficient aggregation strategies and proper
   equipment configuration.  Looking farther forward, there is a clear
   need for better multi-homing technology that does not require global
   routing state for each site and for methods of performing traffic
   load balancing that do not require adding even more state.  Without
   such developments and in the absence of major architectural change,
   aggregation is the only tool available for making routing scale in
   the global Internet.

11.  Status Updates to CIDR Documents

   This memo renders obsolete and requests re-classification as Historic
   the following RFCs describing CIDR usage and deployment:

   o  RFC 1467: Status of CIDR Deployment in the Internet

      This Informational RFC described the status of CIDR deployment in
      1993.  As of 2005, CIDR has been thoroughly deployed, so this
      status note only provides a historical data point.

   o  RFC 1481: IAB Recommendation for an Intermediate Strategy to
      Address the Issue of Scaling

      This very short Informational RFC described the IAB's endorsement
      of the use of CIDR to address scaling issues.  Because the goal of
      RFC 1481 has been achieved, it is now only of historical value.

   o  RFC 1482: Aggregation Support in the NSFNET Policy-Based Routing

      This Informational RFC describes plans for support of route
      aggregation, as specified by CIDR, on the NSFNET.  Because the
      NSFNET has long since ceased to exist and CIDR has been
      ubiquitously deployed, RFC 1482 now only has historical relevance.

   o  RFC 1517: Applicability Statement for the Implementation of
      Classless Inter-Domain Routing (CIDR)

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      This Standards Track RFC described where CIDR was expected to be
      required and where it was expected to be (strongly) recommended.
      With the full deployment of CIDR on the Internet, situations where
      CIDR is not required are of only historical interest.

   o  RFC 1518: An Architecture for IP Address Allocation with CIDR

      This Standards Track RFC discussed routing and address aggregation
      considerations at some length.  Some of these issues are
      summarized in this document in section Section 3.1.  Because
      address assignment policies and procedures now reside mainly with
      the RIRs, it is not appropriate to try to document those practices
      in a Standards Track RFC.  In addition, [RFC3221] also describes
      many of the same issues from point of view of the routing system.

   o  RFC 1520: Exchanging Routing Information Across Provider
      Boundaries in the CIDR Environment

      This Informational RFC described transition scenarios where CIDR
      was not fully supported for exchanging route information between
      providers.  With the full deployment of CIDR on the Internet, such
      scenarios are no longer operationally relevant.

   o  RFC 1817: CIDR and Classful Routing

      This Informational RFC described the implications of CIDR
      deployment in 1995; it notes that formerly-classful addresses were
      to be allocated using CIDR mechanisms and describes the use of a
      default route for non-CIDR-aware sites.  With the full deployment
      of CIDR on the Internet, such scenarios are no longer
      operationally relevant.

   o  RFC 1878: Variable Length Subnet Table For IPv4

      This Informational RFC provided a table of pre-calculated subnet
      masks and address counts for each subnet size.  With the
      incorporation of a similar table into this document (see Section
      3.1), it is no longer necessary to document it in a separate RFC.

   o  RFC 2036: Observations on the use of Components of the Class A
      Address Space within the Internet

      This Informational RFC described several operational issues
      associated with the allocation of classless prefixes from
      previously-classful address space.  With the full deployment of
      CIDR on the Internet and more than half a dozen years of
      experience making classless prefix allocations out of historical
      "Class A" address space, this RFC now has only historical value.

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12.  Security Considerations

   The introduction of routing protocols that support classless prefixes
   and a move to a forwarding model that mandates that more-specific
   (longest-match) routes be preferred when they overlap with routes to
   less-specific prefixes introduces at least two security concerns:

   1.  Traffic can be hijacked by advertising a prefix for a given
       destination that is more specific than the aggregate that is
       normally advertised for that destination.  For example, assume
       that a popular end system with the address is
       connected to a service provider that advertises
       A malicious network operator interested in intercepting traffic
       for this site might advertise, or at least attempt to advertise, into the global routing system.  Because this
       prefix is more specific than the "normal" prefix, traffic will be
       diverted away from the legitimate end system and to the network
       owned by the malicious operator.  Prior to the advent of CIDR, it
       was possible to induce traffic from some parts of the network to
       follow a false advertisement that exactly matched a particular
       network number; CIDR makes this problem somewhat worse, since
       longest-match routing generally causes all traffic to prefer
       more-specific routes over less-specific routes.  The remedy for
       the CIDR-based attack, though, is the same as for a pre-CIDR-
       based attack: establishment of trust relationships between
       providers, coupled with and strong route policy filters at
       provider borders.  Unfortunately, the implementation of such
       filters is difficult in the highly de-centralized Internet.  As a
       workaround, many providers do implement generic filters that set
       upper bounds, derived from RIR guidelines for the sizes of blocks
       that they allocate, on the lengths of prefixes that are accepted
       from other providers.  Note that "spammers" have been observed
       using this sort of attack to hijack address space temporarily in
       order to hide the origin of the traffic ("spam" email messages)
       that they generate.

   2.  Denial-of-service attacks can be launched against many parts of
       the Internet infrastructure by advertising a large number of
       routes into the system.  Such an attack is intended to cause
       router failures by overflowing routing and forwarding tables.  A
       good example of a non-malicious incident that caused this sort of
       failure was the infamous "AS 7007" event [7007], where a router
       mis-configuration by an operator caused a huge number of invalid
       routes to be propagated through the global routing system.
       Again, this sort of attack is not really new with CIDR; using
       legacy Class A/B/C routes, it was possible to advertise a maximum
       of 16843008 unique network numbers into the global routing
       system, a number that is sufficient to cause problems for even

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       the most modern routing equipment made in 2005.  What is
       different is that the moderate complexity of correctly
       configuring routers in the presence of CIDR tends to make
       accidental "attacks" of this sort more likely.  Measures to
       prevent this sort of attack are much the same as those described
       above for the hijacking, with the addition that best common
       practice is also to configure a reasonable maximum number of
       prefixes that a border router will accept from its neighbors.

   Note that this is not intended to be an exhaustive analysis of the
   sorts of attacks that CIDR makes easier; a more comprehensive
   analysis of security vulnerabilities in the global routing system is
   beyond the scope of this document.

13.  Acknowledgements

   The authors wish to express appreciation to the other original
   authors of RFC 1519 (Kannan Varadhan, Jessica Yu); to the ROAD group,
   with whom many of the ideas behind CIDR were inspired and developed;
   and to the early reviewers of this re-spun version of the document
   (Barry Greene, Danny McPherson, Dave Meyer, Eliot Lear, Bill Norton,
   Ted Seely, Philip Smith, Pekka Savola), whose comments, corrections,
   and suggestions were invaluable.  We would especially like to thank
   Geoff Huston for contributions well above and beyond the call of

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14.  References

14.1.  Normative References

   [RFC791]   Postel, J., "Internet Protocol", STD 5, RFC 791, September

14.2.  Informative References

   [7007]     "NANOG mailing list discussion of the "AS 7007" incident",

   [CBGP]     "Graph: Active BGP Table Entries, 1988 to Present",

   [CPOL]     "CIDR Police - Please Pull Over and Show Us Your BGP",

   [CRPT]     "The CIDR Report", <http://www.cidr-report.org/>.

   [IANA]     "Internet Assigned Numbers Authority",

   [LWRD]     "The Long and Winding Road",

   [NRO]      "Number Resource Organization", <http://www.nro.net>.

   [RFC904]   Mills, D., "Exterior Gateway Protocol formal
              specification", RFC 904, April 1 1984.

   [RFC1058]  Hedrick, C., "Routing Information Protocol", RFC 1058,
              June 1988.

   [RFC1195]  Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
              dual environments", RFC 1195, December 1990.

   [RFC1338]  Fuller, V., Li, T., Yu, J., and K. Varadhan,
              "Supernetting: an Address Assignment and Aggregation
              Strategy", RFC 1338, June 1992.

   [RFC1380]  Gross, P. and P. Almquist, "IESG Deliberations on Routing
              and Addressing", RFC 1380, November 1992.

   [RFC1518]  Rekhter, Y. and T. Li, "An Architecture for IP Address
              Allocation with CIDR", RFC 1518, September 1993.

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RFC 4632                 CIDR Address Strategy               August 2006

   [RFC1519]  Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless
              Inter-Domain Routing (CIDR): an Address Assignment and
              Aggregation Strategy", RFC 1519, September 1993.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol", RFC
              2131, March 1997.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

   [RFC2317]  Eidnes, H., de Groot, G., and P. Vixie, "Classless IN-
              ADDR.ARPA delegation", BCP 20, RFC 2317, March 1998.

   [RFC2453]  Malkin, G., "RIP Version 2", STD 56, RFC 2453, November

   [RFC3021]  Retana, A., White, R., Fuller, V., and D. McPherson,
              "Using 31-Bit Prefixes on IPv4 Point-to-Point Links", RFC
              3021, December 2000.

   [RFC3221]  Huston, G., "Commentary on Inter-Domain Routing in the
              Internet", RFC 3221, December 2001.

   [RFC4116]  Abley, J., Lindqvist, K., Davies, E., Black, B., and V.
              Gill, "IPv4 Multihoming Practices and Limitations", RFC
              4116, July 2005.

   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, January 2006.

   [RIPE]     "RIPE Network Coordination Centre", <http://www.ripe.net>.

Authors' Addresses

   Vince Fuller
   170 W. Tasman Drive
   San Jose, CA  95134

   EMail: vaf@cisco.com

   Tony Li
   555 Del Rey Avenue
   Sunnyvale, CA 94085

   Email: tli@tropos.com

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Full Copyright Statement

   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

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©2018 Martin Webb