4. Broadcast Classes

There are several classes of IP broadcasting:

• Single-destination datagrams broadcast on the local hardware net: A datagram is destined for a specific IP host, but the sending host broadcasts it at the data link layer, perhaps to avoid having to do routing. Since this is not an IP broadcast, the IP layer is not involved, except that a host should discard datagram not meant for it without becoming flustered (i.e., printing an error message).

• Broadcast to all hosts on the local hardware net: A distinguished value for the host-number part of the IP address denotes broadcast instead of a specific host. The receiving IP layer must be able to recognize this address as well as its own. However, it might still be useful to distinguish at higher levels between broadcasts and non-broadcasts, especially in gateways. This is the most useful case of broadcast; it allows a host to discover gateways without wired-in tables, it is the basis for address resolution protocols, and it is also useful for accessing such utilities as name servers, time servers, etc., without requiring wired-in addresses.

• Broadcast to all hosts on a remote hardware network: It is occasionally useful to send a broadcast to all hosts on a non-local network; for example, to find the latest version of a hostname database, to bootload a host on a subnet without a bootserver, or to monitor the timeservers on the subnet. This case is the same as local-network broadcasts; the datagram is routed by normal mechanisms until it reaches a gateway attached to the destination hardware network, at which point it is broadcast. This class of broadcasting is also known as "directed broadcasting", or quaintly as sending a "letter bomb" [1].

• Broadcast to all hosts on a subnetted IP network (Multi-subnet broadcasts): A distinguished value for the subnet-number part of the IP address is used to denote "all subnets". Broadcasts to all hosts of a remote subnetted IP network are done just as directed broadcasts to a single subnet.

• Broadcast to the entire Internet: This is probably not useful, and almost certainly not desirable.

For reasons of performance or security, a gateway may choose not to forward broadcasts; especially, it may be a good idea to ban broadcasts into or out of an autonomous group of networks.