[#netfmdst]: Traffic destinations

Traffic Destinations

[#bdcstdoc]: Documentation/standards related to a “broadcast address”

Broadcast Addresses: Documented Standards

There have been some standards/specifications regarding broadcast addresses. (This topic is included for thoroughness, and may exceed the details needed for a simple/basic guide.)

For IPv4, this was theoretically used to communicate with all devices on the local link. Routers tend to listen to such traffic, but then not to route/forward such traffic (with possible exceptions, namely if bridging).

OpenBSD manual page for dhcp-options notes, “Legal values for broadcast addresses are specified in section 3.2.1.3 of RFC 1122.” (That discussion may be referring to text on page 30 of RFC 1122: Internet Layer. This is discussed further in the glossary entry for “network ID”.)

Broadcast traffic is discussed on page 30 of RFC 1122: Internet Layer and on page 31. First, on page 30, the page describes that a notation of negative one ("-1") represents “a field that contains all” bits having a current value that is set to one (not zero). Fully understanding the notation described at the top of page 30 (which is currently covered a bit more in the section about IPv4 0/8) allows an understanding that points c through f apply to broadcast traffic. For information on the less directed broadcast forms, see 255.255.255.255 : The broadcast address. RFC 1122 page 30's points “(d)”, and “e”, and “(f)” all describe other broadcast traffic, and all describe a destination in the first sentence.

Side note: If all of the bits of a host address are cleared to zero, there has been an old standard that would indicate those bits may be intended for broadcast to a subnet. This is covered more in the topic about network ID.