Wild Card Mask

The wild card mask or inverse mask is used to tell the routing protocol which part of the IP address is significant. In Cisco IOS systems they are used in routing protocols such as EIGRP, OSPF, etc… and Access Control List (ACL).

Example 1

This is an example of an IP address with a wild card mask.
Ok, but what does it mean?

Where ever there is a 0 (zero) on the wild card mask, it means this is a significant bit and has to be looked at.

• Look at 113
• Look at 2
• Look at 34

Where ever we see a 255 on the wild card mask, means do not care.

Do not care about 0

So in the example above the routing protocol will look at the first three octets, 113.2.34, only.
If the command such as “network 113.2.34.0 0.0.0.255” is used on Cisco IOS, the routing protocol will be activated on every interfaces with IP addresses starting with 113.2.34

Example 2

This is an IP address with a wild card mask of all zero.

• Every octet is significant
• There has to be an exact match

Example 3

• The first two octets are significant (113.2)
• The last two are not looked at

Note: please make sure you understand that the wild card mask do not influence how a network is being advertised, meaning what the advertised mask or the slash (/..) will be. In the example above, if the network was 113.2.34.0 /30, the /30 will be gossiped across to the neighbour regardless of the wild card mask.