The prefix length is another way of expressing the subnet mask. The prefix length is the number of bits set to 1 in the subnet mask. It is written in “slash notation”, a “/” followed by the number of bits set to 1. For example, if the subnet mask is 255.255.255.0, there are 24 bits set to 1 in the binary version of the subnet mask, so the prefix length is 24 bits or /24. The prefix and the subnet mask are different ways of representing the same thing - the network portion of an address.
Networks are not always assigned a /24 prefix. Depending on the number of hosts on the network, the prefix assigned may be different. Having a different prefix number changes the host range and broadcast address for each network.
The figures illustrate different prefixes using the same 10.1.1.0 address. Figure 1 illustrates /24 to /26 prefixes. Figure 2 illustrates /27 to /28 prefixes.
Notice that the network address could remain the same, but the host range and the broadcast address are different for the different prefix lengths. In the figures, you can see that the number of hosts that can be addressed on the network also changes.