[#brodcast]:

Broadcast Traffic

[#brodaddr]:

Some broadcast addresses

Broadcast in IPv4

For IPv4, see: IPv4 broadcast address (255.255.255.255/32). See also: glossary: broadcast address.

Note that the first address of each subnet is generally considered to be called a network ID, which is the first address of a network. Historically, the network ID has been used for broadcast traffic.

RFC 894: “A Standard for the Transmission of IP Datgrams over Ethernet Networks” states, “The broadcast Internet address (the address on that network with a host part of all binary ones) should be mapped to the broadcast Ethernet address (of all binary ones, FF-FF-FF-FF-FF-FF hex).” (Actually, the phrase “that network” seems to be referring to any individual network, so this is really true of every address “with a host part of all binary ones”, which is the last address of a subnet.)

What that basically means is that the last address of an IPv4 network, including all subnets, is treated as the “broadcast address” of that subnet. Whenever a particular subnet size is being used, then the last address of that subnet should not be used for unicast communications. Such an address is often called “unusable”. The concept of being “unusable” is discussed further by the (glossary entry) section for the phrase “network ID”.

Multicast to all devices on an IPv6 subnet

In an effort to encourage network technicians to become familiar with multicast, some people have stated that IPv6 does not use broadcast, and instead relies on multicast. This is well supported by RFC 4291 page 3 which says “There are no broadcast addresses in IPv6, their function being superseded by multicast addresses.”

Well, if traffic is going to all devices on a subnet, that really does fit the description of “broadcast” traffic. It is true, though, that IPv6 does not use IPv4's method of implementing “broadcast”, and it is true that IPv6 does use multicast to accomplish this type of communication. RFC 4291 page 3's statement about “no broadcast addresses in IPv6” seems to just be indicating that no addresses are given an official classification called a “broadcast” address, because there are addresses that implement the same general purpose as a “broadcast address”, but those addresses (like FF01::1 and FF02::1) are officially categorized as “multicast” addresses. (Further discussion about those addresses RFC 4r291 section 2.7.1.)

For IPv6, see multicast. Specifically, see ff02::1/128 (and ff01::1/128).

Broadcast traffic for Ethernet

There is also a standard for broadcasting Ethernet packets, by using FF-FF-FF-FF-FF-FF. Like IPv4's more commonly used standard for a broadcast ID, this address involves all of the address's bits being “set” (meaning that they have a value of one, and are not cleared to a value of zero).

This Ethernet address is noted by at least one RFC document: As noted in the section about IPv4, RFC 894: “A Standard for the Transmission of IP Datgrams over Ethernet Networks” has a comment that refers “to the broadcast Ethernet address (of all binary ones, FF-FF-FF-FF-FF-FF hex).”

[#bdcstdoc]:

Documentation/standards related to a “broadcast address”

Further information about documentation/standards documents is available, but has been placed on a seperate page. That level of technical detail is beyond what is needed for a general understanding of the concept of broadcast traffic. However, for anyone wanting to see further details that have been written by the established technical experts who made the standards, such information is available in the section about “broadcast traffic” documentation/standards.