Applicability Statement for the Implementation of Classless Inter-Domain Routing (CIDR)
Network Working Group Internet Engineering Steering Group
Request for Comments: 1517 R. Hinden, Editor
Category: Standards Track September 1993
Applicability Statement for the Implementation of
Classless Inter-Domain Routing (CIDR)
Status of this Memo
This RFC specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" for the standardization state and status
of this protocol. Distribution of this memo is unlimited.
As the Internet has evolved and grown in recent years, it has become
clear that it will soon face several serious scaling problems. These
- 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 a mid-sized
organization. Class-C, with a maximum of 254 host addresses, is
too small, while class-B, which allows up to 65534 addresses,
is too large to be densely populated. The result is inefficient
utilization of class-B network numbers.
- Routing information overload. The size and rate of growth of the
routing tables in Internet routers is beyond the ability of
current software (and people) to effectively manage.
- Eventual exhaustion of IP network numbers.
It has become clear that the first two of these problems are likely
to become critical in the near term. Classless Inter-Domain Routing
(CIDR) ttempts to deal with these problems by defining a mechanism to
slow the growth of routing tables and reduce the need to allocate new
IP network numbers. It does not attempt to solve the third problem,
which is of a more long-term nature, but instead 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-
The IESG, after a thorough discussion in the IETF, in June 1992
selected CIDR as the solution for the short term routing table
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RFC 1517 CIDR Applicability Statement September 1993
explosion problem .
2. Components of the Architecture
The CIDR architecture is described in the following documents:
- "An Architecture for IP Address Allocation with CIDR" 
- "Classless Inter-Domain Routing (CIDR): An Address Assignment
and Aggregation Strategy" 
The first of these documents presents the overall architecture of
CIDR; the second describes the specific address allocation scheme to
In addition to these two documents, "Guidelines for Management of IP
Address Space"  provides specific recommendations for assigning IP
addresses that are consistent with  and , and "Status of CIDR
Deployment in the Internet"  describes the timetable for deploying
 in the Internet. Both  and  should be viewed as
supporting, rather than defining, documents.
In addition to the documents mentioned above, CIDR requires that
inter-domain routing protocols be capable of handling reachability
information that is expressed solely in terms of IP address prefixes.
While several inter-domain routing protocols are capable of
supporting such functionality, this Applicability Statement does not
mandate the use of a particular one.
Although Internet routing domains are not required to use routing
protocols capable of propagating CIDR routes, the topology such
routing domains can support will be somewhat limited. In particular,
the non-CIDR-capable parts of the Internet will need to default
towards the CIDR-capable parts of the Internet for routes which have
been aggregated to non-network boundaries.
3. Applicability of CIDR
The CIDR architecture is applicable to any group of connected domains
that supports IP version 4  . CIDR does not require all of the
domains in the Internet to be converted to use CIDR. It assumes that
some of the existing domains in the Internet will never be able to
convert. Despite this, CIDR will still provide connectivity to such
places, although the optimality of routes to these places may be
This Applicability Statement requires Internet domains providing
backbone and/or transit service to fully implement CIDR in order to
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