iptables POSTROUTING not working in local area

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http://serverfault.com – I used command iptables -t nat -I POSTROUTING -o $INTERFACE -p tcp -j SNAT --to-source $IP to make my server packets visible as $IP. But the problem is, that it didn't work inside local area, so when I'm sending something to address 10.X.X.X, then I'm not visible as $IP, but as older IP. EDIT: There are 3 interfaces: auto eth0 iface eth0 inet static address 46.X.X.152 netmask network 46.X.X.0 broadcast 46.X.X.255 gateway 46.X.X.254 post-up /sbin/ifconfig eth0:0 178.X.X.28 netmask broadcast 178.X.X.28 post-down /sbin/ifconfig eth0:0 down (HowTos)