RFC 1517

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

okay bestie, this is RFC 1517 💖 It's a Standards Track doc from September 1993, written by R. Hinden as Editor for the Internet Engineering Steering Group (IESG), sitting in the Network Working Group. The header doesn't list any Obsoletes or Updates, so it's not replacing or changing anything else. It's giving fresh, clean slate energy 🦋

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Status of This Memo 📜

This RFC is an Internet standards track protocol for the Internet community, and it asks for discussion and suggestions for improvements. For the current standardization state and status of this protocol, check the current edition of the "Internet Official Protocol Standards" 📚 Distribution of this memo is unlimited, so share it with everyone 🌸

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1. Introduction ✨

so the Internet has been growing like crazy lately, and it's about to run into some serious scaling problems 😭 There are three of them:

• Exhaustion of the class-B network address space. The root issue is that there's no network class in the middle. Class-C has a max of 254 host addresses, which is way too small, while class-B allows up to 65534 addresses, which is too big to be densely populated. So class-B network numbers end up being used inefficiently 🙈
• Routing information overload. The size and growth rate of the routing tables in Internet routers is beyond what current software (and people) can effectively manage 💀
• Eventual exhaustion of IP network numbers.

the first two are likely to get critical in the near term. Classless Inter-Domain Routing (CIDR) tries to fix those by defining a mechanism that slows the growth of routing tables and reduces the need to allocate new IP network numbers 🧚 It doesn't try to solve the third one, since that's more of a long-term thing. Instead, it eases enough of the short to mid-term difficulties so the Internet keeps running efficiently while we work on a longer-term solution 💜

In June 1992, the IESG, after a thorough discussion in the IETF, picked CIDR as the solution for the short-term routing table explosion problem [1] ✨

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2. Components of the Architecture 🧩

The CIDR architecture is described in these docs:

• "An Architecture for IP Address Allocation with CIDR" [2]
• "Classless Inter-Domain Routing (CIDR): An Address Assignment and Aggregation Strategy" [3]

The first one lays out the overall CIDR architecture, and the second describes the specific address allocation scheme to use 💅

On top of those two, "Guidelines for Management of IP Address Space" [4] gives specific recommendations for assigning IP addresses that match [2] and [3]. And "Status of CIDR Deployment in the Internet" [5] describes the timetable for deploying [4] in the Internet. Both [4] and [5] are supporting docs, not defining ones, so they're more like the hype squad than the main event 🫶

CIDR also requires inter-domain routing protocols to be able to handle reachability information that's expressed purely in terms of IP address prefixes. Several inter-domain routing protocols can do this, but this Applicability Statement doesn't mandate any particular one 🌈

Routing domains aren't required to use routing protocols that can propagate CIDR routes, but ngl the topology they can support gets pretty limited 🥲 Specifically, the non-CIDR-capable parts of the Internet will need to default toward the CIDR-capable parts for routes that have been aggregated to non-network boundaries.

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3. Applicability of CIDR 🌍

CIDR applies to any group of connected domains that supports IP version 4 [6] [7] 🍓 It doesn't require every domain on the Internet to convert, and it assumes some existing domains will never be able to. Even so, CIDR will still provide connectivity to those places, though the optimality of routes to them may be impacted. It's not perfect for everyone, but nobody gets left out totally 🫶

This Applicability Statement requires Internet domains that provide backbone and/or transit service to fully implement CIDR. The point is to make sure the growth of resources routers need for Internet-wide connectivity is significantly slower than the growth of assigned networks 👑

It also strongly recommends that all non-backbone/transit Internet domains implement CIDR, because it reduces the amount of routing information inside those domains 🎀

Individual domains are free to pick whatever inter-domain and intra-domain routing architectures best meet their requirements 🦋 This Applicability Statement doesn't stop a domain, or a group of domains, from using addressing schemes that don't conform to CIDR. Subject to available router resources, CIDR should be able to co-exist with other addressing schemes without hurting overall connectivity. Main character energy for everyone 💖

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4. References 📚

[1] Gross, P., and P. Almquist, "IESG Deliberations on Routing and Addressing", RFC 1380, IESG Chair, IESG Internet AD, November 1992.

[2] Rekhter, Y., and T. Li, "An Architecture for IP Address Allocation with CIDR", RFC 1518, T.J. Watson Research Center, IBM Corp., cisco Systems, September 1993.

[3] Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless Inter-Domain Routing (CIDR): An Address Assignment and Aggregation Strategy", RFC 1519, BARRNet, cisco, Merit, and OARnet, September 1993.

[4] Gerich, E., "Guidelines for Management of IP Address Space", RFC 1466, Merit, May 1993.

[5] Topolcic, C., "Status of CIDR Deployment in the Internet", RFC 1467, CNRI, August 1993.

[6] Postel, J., "Internet Protocol - DARPA Internet Program Protocol Specification", STD 5, RFC 791, USC/Information Sciences Institute, September 1981.

[7] Braden, R., Editor, "Requirements for Internet Hosts -- Communication Layers", STD 3, RFC 1122, IETF, October 1989.

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5. Security Considerations 🔒

Security issues are not discussed in this memo. 💀

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6. Author's Address 📮

Robert M. Hinden
Sun Microsystems
2550 Garcia Ave, MS MTV5-44
Mt. View, CA 94043

Phone: (415) 336-2082
Fax:   (415) 336-6015

EMail: hinden@eng.sun.com

periodt, that's the whole spec 💅🌸