If you want block, unblock...etc
Normal, almost OS Unix as:
Solaris/Solaris-x86
SunOS
NetBSD
FreeBSD
BSD/OS-1.1 - 4
IRIX
OpenBSD
HP-UX 11.00 (IPFilter 4.0alpha*)
Tru64 5.1a (IPFilter 4.0alpha*)
QNX Port
Use ipf as a network filter! You need read document about ipf at:
http://coombs.anu.edu.au/~avalon/