RFC 4632 / START HERE
CIDR, before the details
CIDR means Classless Inter-domain Routing. A prefix is a block of addresses described by its fixed leading bits.
RFC 4632 explains how IPv4 address blocks can be delegated and summarized into routes. A router compares the packet’s destination with those prefixes and follows the most-specific matching route.
The idea in one picture
32-bit address
Longest match
Route or discard
A simplified picture of the idea; the chapters explain the actual mechanisms.
Why it exists
Old address classes wasted space and grew routing tables. This Best Current Practice consolidates the CIDR plan: explicit prefix lengths, aggregation tied to topology and safe handling of unreachable pieces. It does not solve the finite size of IPv4 or specify all of BGP.
A familiar example
Think of a postal route covering a neighborhood, with a more-specific route for one street. In the fictional block 192.0.2.0/24, an upper-half /25 wins for .200. If a street becomes unreachable, the aggregate needs a discard route rather than sending the packet back in a loop. The analogy is not a route-authority check.
Helpful background
You can start here without reading another RFC. If these ideas are unfamiliar, these short introductions may help.
What you’ll learn next
Begin with chapter one, or choose a topic.
- Read a prefixA prefix describes a block of IPv4 addresses. The number after the slash tells you how many of the 32 address bits stay fixed; the remaining bits vary within the block.→
- Combine aligned blocksAggregation advertises a less-specific prefix that covers several smaller blocks. An exact two-block combination requires equal-sized sibling blocks: adjacent ranges with the same parent bits.→
- Choose the longest matchA forwarding table can contain several prefixes that include the same destination. Longest-prefix matching chooses the matching route with the most fixed leading bits.→
- Discard aggregate holesAn aggregate may remain advertised while one of its component routes is unavailable. If the aggregating router sends that traffic back toward its upstream default, the packet can loop.→
- Follow topology and trustAggregation works best when address delegation and network connectivity line up. Multihomed or moved customers need more-specific routes, and an accepted more-specific route can also divert traffic maliciously.→
This introduction explains RFC 4632 (2006). Read the original RFC ↗