4.2.3.1 IP Broadcast Addresses

For historical reasons, there are a number of IP addresses (some standard and some not) which are used to indicate that an IP packet is an IP broadcast. A router

1. MUST treat as IP broadcasts packets addressed to 255.255.255.255 or { <Network-prefix>, -1 }.

2. SHOULD silently discard on receipt (i.e., do not even deliver to applications in the router) any packet addressed to 0.0.0.0 or { <Network-prefix>, 0 }. If these packets are not silently discarded, they MUST be treated as IP broadcasts (see Section [5.3.5]). There MAY be a configuration option to allow receipt of these packets. This option SHOULD default to discarding them.

3. SHOULD (by default) use the limited broadcast address (255.255.255.255) when originating an IP broadcast destined for a connected (sub)network (except when sending an ICMP Address Mask Reply, as discussed in Section [4.3.3.9]). A router MUST receive limited broadcasts.

4. SHOULD NOT originate datagrams addressed to 0.0.0.0 or { <Network-prefix>, 0 }. There MAY be a configuration option to allow generation of these packets (instead of using the relevant 1s format broadcast). This option SHOULD default to not generating them.

DISCUSSION

In the second bullet, the router obviously cannot recognize addresses of the form { <Network-prefix>, 0 } if the router has no interface to that network prefix. In that case, the rules of the second bullet do not apply because, from the point of view of the router, the packet is not an IP broadcast packet.