A router is any device that acts as a gateway between 2 or more separate networks. In our case, that means that the local network is connected to the Internet, and vice versa. The router ignores traffic local traffic or traffic it is not configured to handle, but when Internet access is attempted from within the local network, it springs into action. With NAT (Network Address Translation), the router shares the Internet connection to all the computers connected to the local network. The router keeps track of the originating local computer and manages the connection with the outside Internet server. If a computer on the Internet attempts a connection with the router or to a local computer past the router, the router denies the connection unless the router was specially configured to allow that connection.

Many routers can treat the wireless LAN (WLAN) as a separate network from the wired LAN. This is often done because wireless networks are easy to break into. The idea is that people can keep their wired computers away from the untrusted wireless network. The router will still share the Internet connection (WAN) to both the wireless and wired LANs, regardless of this configuration.