ipnat problem, I do not understand this

From: mylinux (reallinux_at_nospam.com)
Date: 10/28/03 

Date: Tue, 28 Oct 2003 07:47:05 +0800

#!/sbin/ipnat -f -
#
# THIS EXAMPLE IS WRITTEN FOR IP FILTER 3.3
#
# ppp0 - (external) PPP connection to ISP, address a.b.c.d/32
#
# ed0 - (internal) network interface, address w.x.y.z/32
#
# if we get a different dialup IP address each time, then we would use:
#
#map rtk0 192.168.0.0/24 -> 0/32 proxy port ftp ftp/tcp
map rtk0 192.168.0.0/24 -> 0/32 portmap tcp/udp 20000:60000
map rtk0 192.168.0.0/24 -> 0/32
rdr rtk0 0.0.0.0/0 port 1080 -> 192.168.0.10 port 1080 tcp
#rdr rtk1 0.0.0.0/0 port 80 -> 192.168.0.1 port 3128 tcp
#rdr tun0 192.168.0.1/32 port 80 -> 192.168.0.1 port 80 tcp
#rdr tun0 0.0.0.0/0 port 80 -> 192.168.0.1 port 3128 tcp
~