CentOS - Nov 2009 - 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
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> 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" ?
You have your outgoing traffic NATed .. but you need a PREROUTING rule to 
forward the traffic to your Asterisk server.