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I have two different internet connections coming into our office. Each connection runs into it's own router (both routers are the same make, and both are running Tomato firmware).

Right now, computers on both subnets access the internet fine, but they can't talk from one subnet to the other. What am I doing wrong?

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do you have a physical connection between both of your subnets? –  Mark Hall Nov 24 '11 at 20:17
Each router is connected to it's DSL modem, then they both go into the same switch. –  user1064560 Nov 24 '11 at 20:25
Are all the switch ports on the same VLAN? How do you assign the IP addresses? Which router is the DHCP sever? Are all the computers connected to the switch? –  DrNoone Nov 24 '11 at 20:56
What do your routing tables in both routers look like? Can the routers ping each other? are both doing DHCP? At the moment you have two separate networks on the same switch, so without proper routing, they'll remain two separate networks, and if DHCP is enabled on both, you'll have competing DHCP servers as well. –  MaQleod Nov 24 '11 at 21:11
Why don't you edit the question and add your network topography to it. –  Mark Hall Nov 24 '11 at 21:36
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2 Answers

You need a router C to route between the subnets.

You need the existing routers (default gateways for each subnet) to know about the local route to the other subnet (via this router C). Either by static configuration at each router or by allowing routing protocols so that C can advise the others of the route to the other subnet.

It may be that one of your existing routers can perform this function.

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At layer 2 you have a flat network with all devices on the same switch. At layer 3 you have two subnets, one with a default gateway of one router, and the other with a default gateway of the other router.

In order for these two subnets to communicate they need a router between them. You have three immediate options:

1) Change the switch to a layer 3 switch, and put each subnet into its own VLAN, then give the switch vlan virtual interface an IP in each subnet, and it will route between them. You'll need policy based routing to ensure that each subnet routes out of the right internet connection.

2) Add a router to the network with an interface and IP address in each subnet. As your layer 2 vlan is flat, you could do this with a single connection with two IP addresses on the link. You'll need to add a routing entry to the existing routers to route to the new router for the "other" subnet.

3) Give each of the existing routers an IP address in the other subnet. Each router will then route between the two subnets.

Ultimately, the network design is flawed. Your layer 2 and layer 3 networks should be subnetted the same, so option 1 is ideal.

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