Is it possible to connect 2 networks with same IP range?

Solution 1:

It's very, very ugly. The simplest way is to construct a bridging network with two routers in it, with those two routers connected in the bridging network.

Each bridging router NATs 192.168.1.0/24 to a different /24 inside the bridging network. Assuming 192.168.10.0/24, 192.168.11.0/24, 192.168.3.0/24 are free and 192.168.1.251 is free in each network:

Router 1 works as follows:

1. Router 1 is assigned IP address 192.168.3.1/24 in the bridging network and 192.168.1.251 in its own network.

2. Router 1 has a connection to one of the 192.168.1.0/24 networks and NATs 192.168.1.x to and from 192.168.10.x

3. Router 1 has a route for 192.168.11.0/24 with a next hop of 192.168.3.2 in the bridging network.

Router 2 works as follows:

1. Router 2 is assigned IP address 192.168.3.2/24 in the bridging network and 192.168.1.251 in its own network.

2. Router 2 has a connection to the other of the 192.168.1.0/24 networks and NATs 192.168.1.x to and from 192.168.11.x.

3. Router 2 has a route for 192.168.10.0/24 with a next hop of 192.168.3.1 in the bridging network.

In this way, machines on the 192.168.1.0/24 network connected to router 1 can access machines in the second network by using 192.168.11.0 addresses. Vice-versa for the other network, using 192.168.10.0 addresses.

The default router in each network must be configured to route packets for 192.168.10.0/24 and 192.168.11.0/24 to 192.168.1.251. The routers can be cheap wireless routers running OpenWRT, Tomato, or DD-WRT. These routers can also be used for other purposes. For example, if they're already the default router for their 192.168.1.0/24 network, the configuration is slightly simpler.

While it should be possible to do this with a single router using both SNAT and DNAT, I've never gotten it to work. Others I know have reported similar issues. I believe it's because the ARP system can't handle two distinct ARP entries for the same IP on distinct interfaces.