Deliver to either host value
03The 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.
A forwards the destination-.1 packet to B over the /31 link. The source remains the fictional remote host, 192.0.2.10.
Source for this drawing: RFC 3021 · Read RFC section 3.3 ↗
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.
A destination ending in the one host bit is one router, not a broadcast fan-out.
- Section 3.3 says a packet addressed to either /31 host value MUST be treated as directed to the router with that address. Configuring the /31 mask SHOULD allow generating datagrams to its zero host value.
- The source-address restrictions described in §§3.1–3.3 have explicit exceptions when the originator is a /31 point-to-point endpoint. This is not permission to use a subnet broadcast source on other link types.
- The administrator assigns a prefix unique in the local routing domain. The directly connected router interprets the destination; remote forwarding algorithms are not changed (§2.2).
The first two arrows are separate unicast packets, not a guaranteed request/reply exchange. The later two states show a separately configured remote route into this fictional network. A prefix match alone neither installs a route nor guarantees delivery; ordinary policies and reachability still apply. No public routing of TEST-NET addresses is implied.
Read the RFC section ↗