SNAT & POSTROUTING chain

[vic ismakaev <viclists@xxxxxxxxxxxx>]

Hi!
I've setup an iptables-1.2.7a.
  When I try add new rule in POSTROUTING chain.

[root@xxx 123]# iptables  -A POSTROUTING -p tcp -s 192.168.1.2 -j SNAT -o eth0 
--to-source $REAL_IP
iptables: No chain/target/match by that name

or
[root@xxx 123]# iptables -t nat -A POSTROUTING -p tcp  -j SNAT -o eth0 
--to-source $REAL_IP
iptables: Invalid argument

What is it means?

Regards
Victor V Ismakaev