iptables masq. and gateway

[Ricky Malt <ric@xxxxxxxxxxxxx>]

Hi,
  I am currently using ' Rusty's Three-Line Guide To Masquerading '. i.e.
# ipchains -P forward DENY
# ipchains -A forward -i ppp0 -j MASQ
# echo 1 > /proc/sys/net/ipv4/ip_forward

this is making my linux box as a gateway for all on my lan.	

 1 . How can i achieve this with iptables.
 2 . and how can i restrict a particular ip/host to access this as a gw
     to the internet

 I am new to ip filtering.

Regards