There are two major types of IP broadcast addresses; limited
broadcast and directed broadcast. In addition, there are three
subtypes of directed broadcast: a broadcast directed to a specified
network prefix, a broadcast directed to a specified subnetwork, and a
broadcast directed to all subnets of a specified network.
Classification by a router of a broadcast into one of these
categories depends on the broadcast address and on the router's
understanding (if any) of the subnet structure of the destination
network. The same broadcast will be classified differently by
different routers.
A limited IP broadcast address is defined to be all-ones: { -1, -1 }
or 255.255.255.255.
A network-prefix-directed broadcast is composed of the network prefix
of the IP address with a local part of all-ones or { <Network-
prefix>, -1 }. For example, a Class A net broadcast address is
net.255.255.255, a Class B net broadcast address is net.net.255.255
and a Class C net broadcast address is net.net.net.255 where net is a
byte of the network address.
The all-subnets-directed-broadcast is not well defined in a CIDR
environment, and was deprecated in version 1 of this memo.
As was described in Section [4.2.3.1], a router may encounter certain
non-standard IP broadcast addresses:
0.0.0.0 is an obsolete form of the limited broadcast address
{ <Network-prefix>, 0 } is an obsolete form of a network-prefix-
directed broadcast address.
As was described in that section, packets addressed to any of these
addresses SHOULD be silently discarded, but if they are not, they
MUST be treated according to the same rules that apply to packets
addressed to the non-obsolete forms of the broadcast addresses
described above. These rules are described in the next few sections.
