ipnat.rules

[sonjaya]

dear all 

after i finish add in my kernel ipnat , i use this
sample script :
/etc/ipnat.rules :
map rl0 172.18.5.11/255.255.0.0  -> 0.0.0.0/32 proxy
port ftp ftp/tcp ssh
map rl0 172.18.5.11/255.255.0.0 -> 0.0.0.0/32 portmap
tcp/udp auto

in here my net :

lan--NAT server---internet


my question is :

1.how i must set ipnat.rules only some ip get nat
other can not user .
because if some pc station use the gateway my server
that pcstation get nat .

may be like this :
lan(non-nat)-----|
lan(nat)---------|---NAT Server---INternet

2. how i set the map rule in ipnat.rules , that lan do
not  have righ to nat to directly to proxy .

i'm so sory if my question is basic , because i new in
freebsd
thx






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SONJAYA