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Earlier in this chapter (in the “Understanding Switches” section), I mention that packets whose destination MAC addresses are all ones (FF-FF-FF-FF-FF-FF) are intended to be received by all devices that see the packet. Such packets are called broadcast packets.

The scope of the devices that broadcast packets are intended for is called the broadcast domain. Ordinarily, a switch forwards broadcast packets to all the ports on the switch except the port on which the broadcast packet was received. Thus, the broadcast domain consists of all the devices connected to the switch, either directly or indirectly through another switch.

In many cases, allowing broadcast packets to travel throughout a large network is not a good idea. If the network is large, broadcast packets may consume a significant amount of the total bandwidth available on the network, slowing down other more important traffic.

You may be surprised to discover just how much broadcast traffic actually happens on a large network. The most common type of broadcast packet is an Address Resolution Protocol (ARP) request. ARP is the protocol used to determine the MAC address of a given IP address. If one IP device wants to send a packet to another IP device, the sender needs to know the MAC address of the recipient. So, the sender broadcasts an ARP request, which is essentially the question “Does anyone know the MAC address of this particular IP address? If so, please let me know.”

Reducing the amount of broadcast traffic on a network is a key way to improve the network’s overall performance. One of the best ways to do that is to segment the network in a way that splits up the broadcast domains. There are two ways to do this: by using routers, which are described in the next section, or by using VLANs, which are described later in this chapter, in the “Understanding VLANs” section.