12.2 Broadcasting

In Figure 3.9 we showed four different forms of IP broadcast addresses. We now describe them in more detail.

Limited Broadcast

The limited broadcast address is 255.255.255.255. This can be used as the destination address of an IP datagram during the host configuration process, when the host might not know its subnet mask or even its IP address.

A datagram destined for the limited broadcast address is never forwarded by a router under any circumstance. It only appears on the local cable.

An unanswered question is: if a host is multihomed and a process sends a datagram to the limited broadcast address, should the datagram be sent out each connected interface that supports broadcasting? If not, an application that wants to broadcast out all interfaces must determine all the interfaces on the host that support broadcasting, and send a copy out each interface.

Most BSD systems treat 255.255.255.255 as an alias for the broadcast address of the first interface that was configured, and don't provide any way to send a datagram out all attached, broadcast-capable interfaces. Indeed, two applications that send UDP datagrams out every interface are routed (Section 10.3) and rwhod (the server for the BSD rwho client). Both of these applications go through a similar start-up procedure to determine all the interfaces on the host, and which ones are capable of broadcasting. The net-directed broadcast address corresponding to that interface is then used as the destination address for datagrams sent out the interface.

The Host Requirements RFC takes no stand on the issue of whether a multihomed host should send a limited broadcast out all its interfaces.

Net-directed Broadcast

The net-directed broadcast address has a host ID of all one bits. A class A net-directed broadcast address is netid. 255.255.255, where netid is the class A network ID.

A router must forward a net-directed broadcast by default, but it must also have an option to disable this forwarding.

Subnet-directed Broadcast

The subnet-directed broadcast address has a host ID of all one bits but a specific subnet ID. Classification of an IP address as a subnet-directed broadcast address requires knowledge of the subnet mask. For example, if a router receives a datagram destined for 128.1.2.255, this is a subnet-directed broadcast if the class B network 128.1 has a subnet mask of 255.255.255.0, but it is not a broadcast if the subnet mask is 255.255.254.0 (0xfffffe00).

All-subnets-directed Broadcast

An all-subnets-directed broadcast address also requires knowledge of the destination network's subnet mask, to differentiate this broadcast address from a net-directed broadcast address. Both the subnet ID and the host ID are all one bits. For example, if the destination's subnet mask is 255.255.255.0, then the class B IP address 128.1.255.255 is an all-subnets-directed broadcast. But if the network is not subnetted , then this is a net-directed broadcast.

Current feeling [Almquist 1993] is that this type of broadcast is obsolete. It is better to use multicasting than an all-subnets-directed broadcast.

[Almquist 1993] notes that RFC 922 requires that an all-subnets-directed broadcast be sent to all subnets, but no current routers do so. This is fortunate since a host that has been misconfigured without its subnet mask sends all its "local" broadcasts to all subnets. For example, if the host with IP address 128.1.2.3 doesn't set a subnet mask, then its broadcast address normally defaults to 128.1.255.255. But if the subnet mask should have been set to 255.255.255.0, then broadcasts from this misconfigured host appear directed to all subnets.

The first widespread implementation of TCP/IP, the 4.2BSD system in 1983, used a host ID of all zero bits for the broadcast address. One of the earliest references to the broadcast IP address is IEN 212 [Gurwitz and Hinden 1982], and it proposed to define the IP broadcast address as a host ID of one bits. (IENs are the Internet Experiment Notes, basically predecessors to the RFCs.) RFC 894 [Hornig 1984] commented that 4.2BSD used a nonstandard broadcast address, but RFC 906 [Finlayson 1984] noted that there was no Internet standard for the broadcast address. The RFC editor added a footnote to RFC 906 acknowledging the lack of a standard broadcast address, but strongly recommended that a host ID of all one bits be used as the broadcast address. Although Berkeley adopted the use of all one bits for the broadcast address with 4.3BSD in 1986, some operating systems (notably SunOS 4.x) continued to use the non-standard broadcast address through the early 1990s.