Can I have two LANs with the same IP route to each other

Question

Hi there,
I have a customer who has a televantage voip network on a private 192.68.0.0/24 network but has two sites connected via a wireless tower that wants to enable QOS over between the two sites by using a 2621XM router at each end. I've enabled QOS on both routers for voice and video but haven't deployed this yet since I'm not sure if I need to change one of the sites networks to a different network number since I'll have the routes in place with a 192.168.1.0 on the WAN side. If both remote networks stay with the same number scheme of 192.168.0.0/24 will they be able to find each other when requesting where a remote 192.168.0.0 host is or do I need to change of the offices networks to a 192.168.2.0 network to make this work? Sounds kind of basic but before deploying I wanted to get some input...thanks.

Answer 1

Routers cannot route unless the address blocks for each interface are different.
If you have two sites connected by any kind of link, and you'll use one site as a gateay to the Internet, then you'll need at least four different addresses/ blocks:
The 'b' site LAN, i.e., fa/0/0:192.168.0.0/24
The 'b' site WAN (interconnecting link), i.e., fa0/1: 192.168.1.1/30
The 'a' site WAN (interconnecting link), i.e., fa0/1:192.168.1.2/30
The 'a' side LAN, i.e., fa0/0:192.168.2.0/24
(assumes that your wireless link is acting as a bridge or non-routing link) - the fa0/1 device connects to the wireless devices).
If you use the same router on the 'a' side for the interconnecting link for both the 'a' LAN and the Internet gateway, then the third interface would be your Internet WAN address.
The 'b' side LAN interface is your 'b' side default gateway for you clients
The 'a' side LAN interface is the 'a' side default gateway for your clients
Set a default gateway on the 'a' router to the ISPs Default gateway / next hop address.
If you're using a third router for an Internet gateway, then the LAN interface address becomes the default gateway for the clients on that LAN, and you'll need to set up a static route pointing to the other LAN in the routers that connect the two LANs..
A routing protocol may be useful but, depending on your actual bandwidth available, it's probably better to just set up static routes (no routing protocol bandwidth utilization).
The /30 for the WAN link gives some economy of address usage, but it may also secure the link better, since there are no additional addresses for the intruder to use; they'd have to spoof one of the two valid addresses (.1 & .2) as well as crack your encryption (and / or other security).
There's an armload of other possible configurations / topologies / address schemes, I believe this one would be reasonably common.
The bottom line is that a router can only route from one address block to another. In order to pass traffic between two segments with the same address block, you need a bridge / switch, or some other layer one/two device (like a wireless transceiver pair).
Good Luck
Scott