Router

In the OSI reference model, routers operate at Layer 3 (Network Layer) and play a critical role in determining the communication paths for IP packets.

Routers decide on the optimal communication path by referencing their internal routing tables. A routing table contains a list of destination routes that assist the router in determining where to forward incoming packets. If the destination address of a packet matches a route within the table, the router forwards the packet to the next corresponding destination. This process is carried out by multiple routers in a "relay-like" manner, enabling packets to reach hosts on different network segments.

If the packet's destination address does not match any route in the routing table, the router discards the packet and sends a destination unreachable notification to the sender via an ICMP packet. When multiple matching routes exist in the table, the router forwards the packet along the route deemed most optimal.

There are two primary methods to add destination routes to a routing table:

• Static Routing: Destination routes are manually set by administrators.
• Dynamic Routing: The router automatically populates the table by collecting information from neighboring routers. This relies on protocols known as routing protocols to exchange routing data between routers.