Wildcard Masking
Wildcards are used with access lists to specify an individual host, a network, or a certain range of
a network or networks. To understand a wildcard, you need to understand what a block size is;
it’s used to specify a range of addresses. Some of the different block sizes available are 64, 32,
16, 8, and 4.
//When you need to specify a range of addresses, you choose the next-largest block size for your
needs. For example, if you need to specify 34 networks, you need a block size of 64. If you want
to specify 18 hosts, you need a block size of 32. If you only specify 2 networks, then a block size
of 4 would work.
//Wildcards are used with the host or network address to tell the router a range of available
addresses to filter. To specify a host, the address would look like this:
//172.16.30.5 0.0.0.0
//The four zeros represent each octet of the address. Whenever a zero is present, it means that octet
in the address must match exactly. To specify that an octet can be any value, the value of 255 is
used. As an example, here’s how a /24 subnet is specified with a wildcard:
//172.16.30.0 0.0.0.255
//This tells the router to match up the first three octets exactly, but the fourth octet can be any
value.