Separate limited broadcast from directed broadcast
04A 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.
A selected classless routing protocol can use local unicast from .0 to .1. Its protocol payload and other communication modes are outside this illustration.
Source for this drawing: RFC 3021 · Read RFC section 2.2 ↗
New to this? Start hereA quick picture + helpful background
A message needs an address.
Think of a packet as a small addressed envelope. Devices pass it onward; transport protocols decide what delivery promises an application gets.
Helpful first, if these ideas are unfamiliar
These pictures are analogies to help you learn. The linked RFC explains the actual rules.
Terms in this drawing
Related RFCs and changes
“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.
If broadcast traffic is needed on the /31 link, use limited broadcast.
- The limited broadcast MUST be used for all broadcast traffic on a point-to-point link with a 31-bit mask. It is 255.255.255.255; .1 in the chosen .0/31 pair is an endpoint, not a substitute broadcast address.
- Section 3.3 specifically requires 255.255.255.255 for broadcast Address Mask Replies on /31 point-to-point links. This is a historical message example, not advice to deploy an address-mask discovery service.
- Classless routing protocols, which carry explicit prefix lengths, can communicate using local unicast, multicast or limited broadcast (§2.3). /31 changes neither their message formats nor their choice of local peer communication mode.
Only two ends exist in this example, even when a packet has a broadcast destination. The drawing does not claim that limited broadcasts are forwarded to remote subnets. Multicast is mentioned as a separate routing-protocol mode, not conflated with broadcast or mandated for every protocol.
Read the RFC section ↗