Your network connects to the Internet through a gateway router that appears as a node on both your own LAN and your ISP's WAN. In order to exchange data between a network computer and the Internet, you must configure the WAN side of the router with addresses and options supplied by your ISP.
The LAN side of the router controls the way your own network handles communication with the Internet. Therefore, the router's LAN-side settings must be compatible with the settings on each computer and other device connected to the LAN.