Problems with Large Broadcast Domains

A large broadcast domain is a network that connects many hosts. A problem with a large broadcast domain is that these hosts can generate excessive broadcasts and negatively affect the network. In Figure 1, LAN 1 connects 400 users that could generate broadcast traffic resulting in:

• Slow network operations due to the significant amount of traffic it can cause

• Slow device operations because a device must accept and process each broadcast packet

The solution is to reduce the size of the network to create smaller broadcast domains in a process called subnetting. These smaller network spaces are called subnets.

In Figure 2 for example, the 400 users in LAN 1 with network address 172.16.0.0 /16 have been divided into two subnets of 200 users each; 172.16.0.0 /24 and 172.16.1.0 /24. Broadcasts are only propagated within the smaller broadcast domains. Therefore a broadcast in LAN 1 would not propagate to LAN 2.

Notice how the prefix length has changed from a /16 to a /24. This is the basis of subnetting; using host bits to create additional subnets.

Note: The terms subnet and network are often used interchangeably. Most networks are a subnet of some larger address block.