[CentOS] Inquiry:How to enable "NAT" on CentOS 5 ?

 Dear All
On my CentOS 5 , I installed the Asterisk 1.4.13 and DECT application
software and then when I want to try for "NAT" I issue as the followings :
#iptables -t nat -A POSTROUTING -s 10.20.30.0/24 -o eth0 -j MASQUERADE
But it didn't get through . So I checked if the "NAT" is enabled on
my CentOS server , as the followings :
#echo "1" > /proc/sys/net/ipv4/ip_forward
But still I cannot try for "NAT" . Can you please let me know which other
setings maybe influenced and need to be checked for enabling the "NAT" ?
Thank you in advance
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.centos.org/pipermail/centos/attachments/20091123/af3b3d85/attachment.html>