RFC 1817

CIDR and Classful Routing

📅 published August 1995, from ietf

RFC 1817: CIDR and Classful Routing 💅

okay bestie, this is RFC 1817, from the Network Working Group 🎀 Here's the title block:

  • Number: 1817
  • Title: CIDR and Classful Routing
  • Author: Yakov Rekhter, cisco Systems
  • Category: Informational
  • Date: August 1995
  • Obsoletes / Updates: the title block doesn't list anything it obsoletes or updates

Status of this Memo 🌸

this memo is just giving info to the internet community ✨ it does not specify an Internet standard of any kind, so it's not a spec you have to follow. distribution of this memo is unlimited, so share it with everyone 🫶

Abstract 🦋

CIDR (Classless Inter-Domain Routing) is used across the Internet as the primary mechanism for improving the scalability of the Internet routing system. this document is the IAB's (Internet Architecture Board) evaluation of what CIDR means, now and in the near future, for organizations that use Classful routing technology. it's giving big-picture analysis 🔥

Background 🧚

okay so CIDR ([RFC1518], [RFC1519]) is deployed across the Internet as the main way to improve the scaling of the Internet routing system 💖 the key ingredients are generalizing the concept of variable length subnet masks (VLSM) and getting rid of the classes of network numbers (A, B, and C). the interior (intra-domain) routing protocols that support CIDR are OSPF, RIP II, Integrated IS-IS, and E-IGRP. the exterior (inter-domain) routing protocol that supports CIDR is BGP-4. protocols like RIP, BGP-3, EGP, and IGRP do not support CIDR 💀

Implications of CIDR 👑

the rollout of CIDR has some real implications for the parts of the Internet still running routing tech that can't support it 😭 sites that depend only on a default route for their external connectivity may not need VLSM-capable routing for their interior routing, or CIDR for their exterior routing. every site lacking support for VLSM and CIDR-capable routing has to rely on a default route, which can lead to a varying degree of suboptimal routing. organizations that operate as Internet Service Providers (ISPs) are expected to be able to support VLSM and CIDR 🌈

it's expected that in the near future the IANA will instruct the Internet Registries to start allocating IP addresses out of the former Class A address space (64.0.0.0 through 126.0.0.0). the allocated blocks will be variable in size, based on each site's actual requirements. sites that use these addresses will have to support CIDR-capable routing protocols, and all the providers will be required to support them too. sites that don't use these addresses would be required to keep relying on a default route, which in turn may result in a varying degree of suboptimal routing. if a site wants to avoid that suboptimality (introduced by using a default route), it will need to transition to CIDR-capable routing protocols ⭐

Security Considerations 💀

security issues are not discussed in this memo. yes, this is a very serious matter, and it's not covered here 🙈 so if you're thinking about security for CIDR deployments, you'll need to look elsewhere 🔥

Author's Address 💜

Yakov Rekhter
cisco Systems
170 West Tasman Drive
San Jose, CA 95134

Phone: (914) 528-0090
EMail: yakov@cisco.com
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