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IP Routing Fundamentals by Mark A. Sportack

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WAN Gateway

Perhaps the most familiar, and important, of a router's LAN-level functions is its use as a gateway to the WAN. This is the function that LAN-based routers were originally designed for. In this original context, a LAN's router was its gateway to the internetwork that lay beyond its border. The router can function in this capacity because it supports operation at Layer 3, the network layer, of the OSI reference model.

The network layer encompasses all the protocols, address architectures, and other mechanisms needed to pass data beyond the domain boundaries of LANs. To utilize network layer protocols requires the use of a networking protocol, such as IP. Networking protocols, known more specifically as either routed or routable protocols, ...

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