We often hear from people who have two (or more!) routers in their LAN and are trying to get Microsoft File and Printer sharing running among all their computers. This ProblemSolver will explain why this doesn't work by default and provide some suggestions for working around the problem.
Figure 1 shows a two-router LAN configuration that I'll use as an example. This isn't the only configuration possible, but it will serve to illustrate the points I'll be making.
First, note that the two routers are set to different base addresses - the wired router to 192.168.1.1 and the wireless router to 192.168.2.1. This is essential for multi-router setups, since without the different address ranges, the routers wouldn't be able to properly build their routing tables. These tables control the way that data is handled and ensure that it is sent to the correct router for delivery to its connected clients. The use of different base addresses puts each router's attached clients into different Class C subnets.
TIP: Class C subnets have a maximum of 254 IP addresses, have the same first three "octets" in their addresses (ex. 192.168.3.X) and use a subnet mask of 255.255.255.0.
Next, note that the second router (the wireless) has its WAN port connected to one of the wired router's LAN ports via a normal UTP patch cable, and that it has an IP address in the first router's range. I've shown the wireless router's WAN IP as 192.168.1.100, but it could be any IP address in the 192.168.1.X subnet.
TIP: You don't have to use the 192.168.X subnets shown in the example. You can use any two private IP address ranges as long as they are different.
Note also that you can either assign the second router's WAN IP statically, or just set it to be a DHCP client (obtain automatically). I suggest the latter option, since if you enter the IP address info manually, you'll need to include the Gateway and DNS information, which you might have trouble figuring out.
Our two-router setup doesn't cause problems with simple Internet use including email, web browsing, instant messaging, and most applications, i.e. anything where you initiate the request for data. But you'll run into two problems when you try to get file and printer sharing going, which I'll now explain.
Problem 1 is that the multiple subnets in our example LAN cause problems with network browsing. This means when you use My Network Places in Win2000 and XP or Network Neighborhood in earlier versions of Windows, the only computers you'll see are those connected to the same router. This problem can be solved by using a WINS server, but there are simpler fixes that I'll describe in the next section.
Problem 2 is caused by each router's firewall. By default, consumer routers block all unrequested data that tries to travel from WAN port (the Internet) to LAN clients, and passes all outbound data from LAN clients to WAN. The blocking of inbound data requests provides the basic "firewalling" function of a NAT router and keeps computers connected to the router's LAN ports inaccessible from the Internet. But this inbound filter gets in the way of Microsoft File and Printer sharing when routers are connected together.
Referring to our example LAN in Figure 1, this means wireless router clients will be able to file and printer share with clients of the wired router, but not vice-versa.
NOTE: This same "one-way" action will also complicate access to servers or server-type applications running on any computers connected to the second router. A simple fix for this problem is to connect those computers to the first router, but the file and printer sharing work-arounds shown later can also be used.