Nat setup when one physical interface?

Question

linux

In general, I have a Linux router on the network, only one physical interface is connected to it and there are 4 virtual ones.
So today it turned out that the MASQUERADE rule turned out to replace all addresses, that is, in the local network, if network A ( 10.0.0.0/24 ) accessed network B ( 172.16.0.0/24 ) then the router replaced the address with the one specified in the iptables rule.
In general, I have not yet been able to figure out how to make sure that local networks are routed in the usual way and that the replacement occurs only when someone tries to access the external network to access the Internet.
I'm looking for solutions and tips

Answer 1

iptables -t nat -A POSTROUTING -s 10.0.0.0/24 ! -d 172.16.0.0/24  -o eth0 -j SNAT --to-source 208.77.188.166
iptables -t nat -A POSTROUTING -s 172.16.0.0/24  ! -d 10.0.0.0/24  -o eth0 -j SNAT --to-source 208.77.188.166

208.77.188.166 - which source can be substituted
instead of "SNAT --to-source 208.77.188.166" MASQUERADE, but SNAT is preferable