There are times when having two separate networks - both sharing the same Internet connection - can come in handy. For example, I recently helped a community center with its network setup. They needed to provide Internet connection to tenants who were renting space, in addition to their own shared Internet. They also shared a number of folders on the network, but weren't too careful about password protecting the shares.
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.