Connecting 2 LANs, each with own DSL router, DHCP server, etc.

I am not completely clear on the method described here.

In my case, we have two separate LANs in a single office suite. Each LAN has its own ISP and router, DHCP server, etc. We now need the 2 LANs to be connected, in order to share an IP phone system, but need to maintain our separate LANs, ISPs, routers and, in general, keep the 2 LANs isolated from each other, except for the ports required for the phone system.

LAN 1 is 192.168.1.x
LAN 2 is 192.168.0.x

Does it make any difference which LAN is connected to the LAN port on "Router C" and which is connected to the WAN port?

Also, suggested routers would be helpful, as I do not think all routers let you choose between NAT routing and classic routing.

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Steve JenningsIT ManagerCommented:
You can get a vyatta software router, run it on a pc with 2 network interface cards and do exactly what you want to do. This keeps things totally separate and all you need is a PC, vyatta is opensource. You can setup routing, firewalling, etc on the vyatta without having to touch anything else on either network.

There are other solutions, certainly, but if you have a spare PC, two ethernet cables, and some time (not much) to learn vyatta this is an inexpensive and efficient solution.

Good luck,
Two other options:

1. Use a Layer3 capable switch to connect the 2 networks and route traffic between them.
2. Us an inexpensive router to do the same.
nxnwAuthor Commented:
Sorry, I should have been more clear  I understand that it can be done with a router, as was explained in the  related question ( ).

What I am looking for are some clarifications to THAT solution, i.e.:

Does it make any difference which LAN is connected to the LAN port on "Router C" and which is connected to the WAN port?

Is it necessary/useful to configure the static routes in "Router C" as well?

Also, suggested routers would be helpful, as I do not think all routers let you choose between NAT routing and classic routing (and there may also be throughput issues).
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If you decide to use a router, you will need to change the default configuration.
On most, NAT and firewall features are enabled; they will need to be turned off.

Once that is done, the concepts of WAN and LAN ports doesn't matter any more. It will just route traffic from one network to the other.
nxnwAuthor Commented:
Thanks for that clarification.

From what I can tell, not all routers let you choose between NAT and classic routing. Also, I want to have the benefit of the firewall, to keep the separate LANs private from each other, except for the ports that required to be open for the phone system.

Is that feasible? Any ideas for a readily available router that will fill the bill?

Given a router that can do this, is any other configuration required for Router C (the router linking the two LANs), besides setting up the port filtering and not using NAT?  
Yes, every router I've seen will let you chose. Some manufacturers have different terminology.
For example, in some routers, turning off 'gateway mode' accomplishes the same thing as turning off NAT, etc.

Keeping the firewall enabled will make it a little trickier, but can be done.

Assuming you're not going to need a lot of bandwidth between the networks, then your basic Linksys/Netgear/DLink etc will work. If you need something with more horsepower look at their business-class routers.

Once the router is in place, the only other thing you'll need to do is to add a static route in each of the existing internet routers pointing to RouterC for the other network.

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nxnwAuthor Commented:
I'm not sure what kind of horsepower is required for IP telephony. I suspect the data is tremendously compressed, though, as the phones apparently work well remotely, connected by VPN tunnels. I guess if I get it to work, and throughput is a problem, at least I'm on the right track.
Your IP telephony bandwidth depends on what codec you're using.
G.711 uses about 84k of bandwidth per call.
The newer G.729 uses < 32k per call.

Most systems still use G.711
For example 3 concurrent phone calls will consume about 252k of bandwidth.

As you can see IP telephony doesn't use a lot of network resources, but is susceptible to jitter and latency. You should ensure you are implementing some type of QoS on your network.
nxnwAuthor Commented:
Sorry I have not closed this off yet. The other network has not decided whether it is prepared to try this solution, so I have not been able to try it out yet.
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