EDIT: Consider looking at and upvoting Malvineous's answer on this page. Netmasks are a much more elegant solution.
Simply use a percent sign as a wildcard in the IP address.
From http://dev.mysql.com/doc/refman/5.1/en/grant.html
You can specify wildcards in the host name. For example,
user_name@'%.example.com'
applies touser_name
for any host in theexample.com
domain, anduser_name@'192.168.1.%'
applies touser_name
for any host in the192.168.1
class C subnet.