NetBSD-5.0.2/dist/ipf/rules/nat.eg

# map all tcp connections from 10.1.0.0/16 to 240.1.0.1, changing the source
# port number to something between 10,000 and 20,000 inclusive.  For all other
# IP packets, allocate an IP # between 240.1.0.0 and 240.1.0.255, temporarily
# for each new user.
#
map ed1 10.1.0.0/16 -> 240.1.0.1/32 portmap tcp 10000:20000
map ed1 10.1.0.0/16 -> 240.1.0.0/24
#
# Redirection is triggered for input packets.
# For example, to redirect FTP connections through this box, to the local ftp
# port, forcing them to connect through a proxy, you would use:
#
rdr ed0 0.0.0.0/0 port ftp -> 127.0.0.1 port ftp
#