ports/net/netmask/pkg-descr

This is a handy tool for generating terse netmasks in several common
formats.  If you've ever maintained a firewall with more than a few rules
in it, you might use netmask to clean up and generalize sloppy rules left by
the netadmin before you.  It will also convert netmasks from one format
to another for the day you change your firewall software.