Find the directed broadcast address
01IPv4 addresses have 32 bits. A directed broadcast keeps a particular network prefix and sets every host bit to one; the last-hop router needs the prefix to recognize it.
255.255.255.255 means limited broadcast on the connected network. It is not a substitute spelling for the chosen network-directed destination.
Source for this drawing: RFC 2644 · Read RFC section 3 ↗
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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.
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Related RFCs and changes
“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.
RFC 2644 §3 defines { <Network-prefix>, -1 }, where -1 means all host bits are one. The router context supplies the prefix; a final .255 alone is not a universal broadcast test.
- RFC 2644 §3 defines { <Network-prefix>, -1 }, where -1 means all host bits are one. The router context supplies the prefix; a final .255 alone is not a universal broadcast test.
- For the chosen 192.0.2.0/24 example, 24 prefix bits leave eight host bits, giving destination 192.0.2.255. For 192.0.2.0/25, the directed broadcast is 192.0.2.127 instead.
- 255.255.255.255 is the separate limited broadcast case described in RFC 1812 §4.2.2.11(c). These scenes do not apply ordinary broadcast arithmetic to /31 point-to-point links or /32 routes.
Fictional documentation addresses and a last-hop router that knows the destination prefix. A bounded teaching illustration, not a router configuration or packet tool.
Read the RFC section ↗