A large number of networks have unknown and unauthorized back doors.
If it's a decent-sized network and you haven't audited it, don't assume
that the firewalling is effective. (My co-author on "Firewalls and
Internet Security" book, Bill Cheswick, is CTO of a startup that maps
intranets for just this reason.)
--Steve Bellovin, http://www.research.att.com/~smb