I have three machines on three networks

192.x.x.x
10.x.x.x
172.x.x.x

The routers are set to forward communication between 192. network and 10. network, and between the 10. network and the 172. network.

However, there's not routing between 192. and 172.

I want to fix that by using a machine on the 10. network to forward communication between the other two networks.

The machine has one etherent connection eth0 whose address is 10.1.1.11
I set up an aliased ip address eth0:0 to be 10.1.1.12 using
Then I tried to set forwarding rules the 10. machine such that 10.1.1.12 address will provide access to the machine 172.1.1.55 as follows
The default policies for all chains is ACCEPT.

I then try to access 10.1.1.12 from 192.1.1.20 expecting it to actually access 172.1.1.55 ; it does not work

What am I missing?

Before we can give you an answer...

1. Do you have three routers with each of these three networks behind them?

For example:

Router 1 inside interface: 192.168.0.0/24
Router 2 inside interface: 10.0.0.0/8
Router 3 inside interface: 172.16.0.0/16

2. If there are three routers, what connects the three routers together? Do they each share an external interface on a common switch with a different address space?

3. Or do you have two routers, each with a 192.x.x.x and 172.x.x.x on their inside interfaces... and both of their outside interfaces connected to a switch with your Linux host on the same 10.x.x.x network.

If this is the case, I'm assuming you're trying to configure what's called router-on-a-stick. In which case, you shouldn't have to NAT, just route. This would also beg the question, why not just add the appropriate routes pointing to the respective outside interfaces on each of the routers?

Yes
Router 1 inside interface: 192.168.0.0/24
Router 2 inside interface: 10.1.1.0/24
Router 3 inside interface: 172.1.1.0/24
Router 1 connects to router 2 over site-to-site vpn
I don't know how router 2 connects to router 3

Found it