If you want to match IP address as well as the subnet mask of a BGP route, you can use extended IP access-lists to match both. The extended access-lists can be used in neighbor distribute-list in/out router configuration command or in a match ip address command within a route-map.

When I've included a few slides on this feature in the first BGP course I've developed for Cisco (that was probably somewhere around 1994), the results in the class were always the same: total confusion that needed an hour of whiteboard examples to dissolve. You can find a few examples that will help you understand this arcane feature in a post written by Brian Dennis.

The use of extended IP ACL as a route matching mechanism was made obsolete by the ip prefix-list command, which was introduced in 12.0T. As 12.0T reached End-of-Engineering in the previous millennium, it's a safe bet that the only place where you might still be required to use extended ACLs to match IP routes is in the CCIE lab.