[CentOS] SNAT

Hello I will briefly draw the situation

Router with one interface eth0 , to local network 10.123.0.0/16

on a local network ADSL modem with IP  10.123.10.11

I want to use 10.123.10.11 as a connection to internet .

Because of that I have created default route "ip route add default via
10.123.10.11 dev eth0"

I do not want my ADSL modem to do NATing , since it shows to be slow.
I have configured static route on ADSL modem "10.123.0.0/16 via
10.123.10.11" which ensures traffic comming from internet will ge to
my router.

Now i want to configure NATTING. My concern is, how to create iptables
rule which will match only the traffic going via 10.123.10.11 and only
for this will make SNAT.
If I will do iptables -A POSTROUTING -o eth0 --j SNAT --to-source
213.194.242.198 this will not work . I need to add there a magic words
that only traffic going via 10.123.10.11 should be SNATted.

Please help,

Thank you in advance.

David