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Broadcast Communication and Addresses

Layer 4 broadcast communication is the process of sending a packet from one host to all hosts in the network. Unlike unicast communication, which uses the destination host address, broadcast and multicast communication use special addresses as the destination address. This special address, called the broadcast address, allows all the receiving hosts to accept the packet. When a host receives a packet with the broadcast address as the destina-tion, it processes the packet as it would a packet to its unicast address. Using these special addresses, broadcasts are generally restricted to the local network.

Broadcast transmission is used for the location of special services/devices for which the address is not known or when a host needs to provide information to all the hosts on the network. Some examples for using broadcast transmission are as follows:

■   Mapping upper-layer addresses to lower-layer addresses

■   Requesting an address

■   Exchanging routing information by routing protocols

When a host needs information, the host sends a request, called a query, to the broadcast address. All hosts in the network receive and process this query. One or more of the hosts with the requested information will respond, typically using unicast. Similarly, when a host needs to send information to the hosts on a network, it creates and sends a broadcast packet with the information.

Unlike unicast, where the packets can be routed throughout the internetwork, broadcast packets are usually restricted to the local network and not forwarded by a router. This restriction is dependent on the configuration of the router that borders the network and the type of broadcast.

There are two types of broadcasts: directed broadcast and limited broadcast. Each of these two types of broadcasts uses a different method of IPv4 addressing.

Directed Broadcast

A directed broadcast is sent to all hosts on a specific network. This type of broadcast is use-ful for sending a broadcast to all hosts on a nonlocal network. Directed broadcast uses an IPv4 destination address that is the highest address in a network. This is the network address with all 1s in the host bits. For example, for a host outside the network to commu-nicate with the hosts within the 172.16.4.0 /24 network, the destination address of the pack-et would be 172.16.4.255. Although routers do not forward directed broadcasts by default, they can be configured to do so.

Limited Broadcast

The limited broadcast is used for communication that is limited to the hosts on the local network. These packets use a destination IPv4 address of all 1s (255.255.255.255). Routers do not forward this broadcast. Packets addressed to the limited broadcast address will only appear on the local network. For this reason, an IPv4 network is also referred to as a broad-cast domain. Routers form the boundary for a broadcast domain. As an example, a host within the 172.16.4.0 /24 network would broadcast to all the hosts in its network using a packet with a destination address of 255.255.255.255.

Figure 6-7 represents a limited broadcast from host A with the address 172.16.4.1. In this example, the source host creates a single packet addressed to the Layer 3 broadcast address. The lower-layer services will use a corresponding data link layer address to forward this