Router

A router is a network device whose main purpose is to facilitate communication between networks. Each interface on a router creates a boundary network.

Routers operate at Layer 3 of the OSI Model, which means they only see each datagram up to the Layer 3 header. The Layer 3 header contains possible information end-to-end delivery, such as Source and Destination IP Addresses.

 

In the image above, notice that the router is on the left (R1) and the router is on the side right (R2) creates three separate networks (11.11.11.x, 22.22.22.x, and 33.33.33.x). R1's right interface and R2's left interface are both on the same network. The only way for Clients on the 11.11.11.x network to talk to Servers on 33.33.33.x network is to forward the packet to R1, which in turn will forwards the packet to R2, which will eventually forward the packet to the Server.

Routers accomplish all this by maintaining what is known as Routing Table (Routing Table). This is a table containing paths to all networks who knows how to be reached by the Router. These paths are sometimes known as Routes, and each entry contains the Network IP and either the interface or IP address of the router next in path to target.

There are several ways a Router can learn from the network and populate its Routing Table. We'll look at some of these ways in the next article in this series.

Keep in mind, from each router's perspective, the Route Table is a map of every existing network. If the router receives a packet addressed to that network unknown, then as far as the router is concerned, the network must not exist. By Therefore, when the router receives a packet addressed to a network that is not
is in its Routing Table, the packet is discarded.