CIDR: IAB Recommendation for an Intermediate Strategy to Address the Issue of Scaling
An emergency plan to extend the life of IPv4 addresses
Huitema, C., "IAB Recommendation for an Intermediate Strategy to Address the Issue of Scaling", RFC 1481, DOI 10.17487/RFC1481, July 1993, <https://www.rfc-editor.org/info/rfc1481>.
@misc{rfc1481,
series = {Request for Comments},
number = 1481,
howpublished = {RFC 1481},
publisher = {RFC Editor},
doi = {10.17487/RFC1481},
url = {https://www.rfc-editor.org/info/rfc1481},
author = {C. Huitema},
title = {{IAB Recommendation for an Intermediate Strategy to Address the Issue of Scaling}},
pagetotal = 2,
year = 1993,
month = jul,
} RFC text
Contents
Network Working Group C. Huitema, Chair
Request for Comments: 1481 Internet Architecture Board
July 1993
IAB Recommendation for an Intermediate Strategy to
Address the Issue of Scaling
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard. Distribution of this memo is
unlimited.
Recommendation
Classless Inter-Domain Routing (CIDR) proposes strategies for address
assignment of the existing IP address space with a view to conserve
the address space and stem the explosive growth of routing tables in
default-route-free routers run by transit routing domain providers
[1]. CIDR is proposed as an immediate term strategy to extend the
life of the current 32 bit IP address space. This strategy presumes
that a suitable long term solution is being addressed within the
Internet technical community.
The basic components of the CIDR plan are: management of the
allocation of Internet address space and provision of a mechanism for
aggregation of routing information. The IP community has published
several RFCs and Internet-Drafts which describe the architecture for
IP address assignment and routing protocols which will promote the
deployment of CIDR. These documents have led to changes in the way
network address are allocated and have prompted enhancements to the
inter-domain and intra-domain routing protocols.
With the CIDR prompted changes in the management of the allocation of
the Internet address space, allocation of blocks of Class C numbers
leads to an explosion of the routing tables. So, it is important
that the techniques for aggregating information in the routing
protocols keep pace with the change in the allocation of IP numbers.
The IAB endorses the CIDR architecture and its implementation. In
addition, the IAB supports the actions taken by the IANA and
InterNIC, the router vendors, and the network operators to implement
CIDR to address the scaling problem that we are facing with the
growth of the Internet.
References
[1] Fuller, V., Li, T., Yu, J., and K. Varadhan, "Supernetting: an
Address Assignment and Aggregation Strategy", RFC 1338, BARRNet,
cisco, Merit, OARnet, June 1992.
[2] Gerich, E., "Guidelines for management of IP Address Space", RFC
1466, Merit, May 1993.
[3] Topolcic, C., "Schedule for IP Address Space Management
Guidelines", RFC 1367, CNRI, October 1992.
[4] Rekhter, Y., and T. Li, "An Architecture for IP Address
Allocation with CIDR", Work in Progress, January 1993.
[5] Rekhter, Y., and C. Topolcic, "Exchanging Routing Information
across Provider/Subscriber Boundaries in CIDR environment", Work
in Progress, February 1993.
Security Considerations
Security issues are not discussed in this memo.
Author's Address
Christian Huitema
INRIA, Sophia-Antipolis
2004 Route des Lucioles
BP 109
F-06561 Valbonne Cedex
France
Phone: +33 93 65 77 15
EMail: Christian.Huitema@MIRSA.INRIA.FR
AI ELI5
On this page
§1The Scaling Crisis
In the early 1990s, the Internet experienced explosive growth. Two problems threatened to break the system. First, the supply of IPv4 addresses—32-bit numbers that uniquely identify computers—was being depleted rapidly. Each computer needed a unique address, and the space of 2^32 possible addresses seemed infinite until demand suddenly grew exponentially.
Second, core routers maintained routing tables that listed how to reach every network reachable from their location. As millions of new networks connected to the Internet, these tables grew from thousands of entries to hundreds of thousands. Storing and searching these giant tables became a bottleneck that threatened network performance.
The Internet needed billions of unique addresses for all the computers connecting to it, but there were only enough for millions. And every router needed to remember how to reach every network, which was becoming impossible.
§2CIDR: The Intermediate Solution
CIDR (Classless Inter-Domain Routing) is a set of practices for assigning and routing IP addresses more efficiently. It has two main parts.
First, address allocation became classless: instead of assigning addresses in fixed-size classes (where a block was either 16 million addresses or 65,536 or 256), the Internet Address Registry could assign a block of any convenient size. This reduced wasted addresses and accommodated networks of any size without waste.
Second, routing tables practice aggregation: if one organization controls a contiguous block of addresses, routers no longer list each subnetwork separately. Instead, they list the entire block with a single entry. This dramatically shrinks routing tables. The IAB endorsed CIDR in 1993, and it was rapidly deployed by router vendors, Internet service providers, and address registries.
CIDR gives out addresses in exactly the sizes needed, and routers remember them in groups instead of individually. One entry covers a thousand addresses instead of listing them all.
§3A Temporary Measure
The memo makes clear that CIDR was always intended as a short- to intermediate-term solution. It extends the life of the 32-bit IPv4 address space but does not solve the fundamental problem: IPv4 addresses are finite and the Internet's growth is exponential.
The long-term solution, endorsed by the Internet community, is IPv6, which uses 128-bit addresses and provides 2^96 times more addresses than IPv4. However, IPv6 is far more complex to deploy. CIDR served its purpose: it bought time for the Internet to grow from millions to billions of users while IPv6 was developed and gradually deployed.
CIDR was meant to be a temporary fix while a bigger solution was being built. That bigger solution is IPv6, which has vastly more addresses.