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RFC 3021 / START HERE

IPv4 /31 links, before the details

The title describes a subnet choice, not an acronym. IPv4 is Internet Protocol version 4; /31 means 31 fixed address bits.

RFC 3021 lets the two ends of an IPv4 point-to-point link use both values in a /31 block as endpoint addresses. A point-to-point link connects exactly two ends; this is a narrow exception to familiar subnet network/broadcast rules.

Start the guide See the 5 chapters ↓

The idea in one picture

  1. Two link endpoints

  2. One host bit

  3. Two usable addresses

A simplified picture of the idea; the chapters explain the actual mechanisms.

Why it exists

It halves a numbered link’s address allocation from four values to two while keeping both endpoints identifiable. It explains host interpretation, broadcast handling and compatibility; it does not change IPv4 packet headers or increase link speed.

A familiar example

A network operator can number a supported two-router link with two addresses rather than four. Both ends need the correct support, and the rule should not be copied onto a shared local-area network (LAN). Addresses in this guide are fictional documentation examples, not a live configuration.

Helpful background

You can start here without reading another RFC. If these ideas are unfamiliar, these short introductions may help.

IPv4RFC 791 · beginner overview ↗Private IPv4 addressesRFC 1918 · beginner overview ↗

What you’ll learn next

Begin with chapter one, or choose a topic.

  1. Spend two addresses on two endpointsA point-to-point link has exactly two ends. A /30 reserves four IPv4 address values; a /31 can number the same two endpoints with two values.
  2. Read the last host bitA slash prefix says how many leading address bits are fixed. IPv4 has 32 bits; a /31 leaves one host bit, which can be 0 or 1.
  3. Deliver to either host valueThe directly connected router applies the /31 endpoint interpretation. Unicast means delivery to one destination. Packets can use either host value as source or destination within the stated exception.
  4. Separate limited broadcast from directed broadcastA directed broadcast targets a subnet’s hosts from elsewhere. A /31 point-to-point subnet has no such address: its two values already identify its two endpoints.
  5. Check both ends before deploymentThe /31 rule is scoped to point-to-point interfaces. Support on only one end can break operation; the address saving is not a universal connectivity or security guarantee.
Start with chapter one →

This introduction explains RFC 3021 (2000). Read the original RFC ↗