How Routers Work: From Routing Tables to Dynamic Path Selection

What a router is

A router (gateway device) connects multiple logically separate networks or subnets. It examines destination IP addresses, selects an IP path, and forwards packets between networks at the network layer (Layer 3).

Routing table

The router maintains a routing table that records, for each destination network, the source, routing entry, and next‑hop address. The table is updated periodically and when topology changes.

Packet‑forwarding process

When a packet arrives, the router looks up the destination IP in the routing table. If a matching entry exists, the packet is forwarded to the indicated next hop; otherwise the packet is dropped and the source host is notified that the destination is unreachable.

How routing entries are obtained

Direct (connected) routes : the router discovers directly attached neighbor networks automatically.

Static routes : manually configured paths, suitable for small networks but costly to maintain as the topology changes.

Dynamic routes : learned automatically via routing protocols; large deployments often combine dynamic and static routes.

Layer‑2 vs Layer‑3 addressing

Switches operate at Layer 2 using MAC addresses. Routers operate at Layer 3 using IP addresses. A host first checks whether the destination IP is in the same subnet. If it is, the packet is forwarded by the switch (Layer 2). If not, the host sends the packet to its default router.

Three‑layer addressing flow

When a host wants to send data, it determines whether the destination is in the same subnet. If yes, the switch performs Layer‑2 forwarding. If no, the host forwards the packet to its router, which consults its routing table and forwards to the next hop.

Multi‑router forwarding example

When the destination is in a different subnet, the packet may traverse several routers, e.g., Router A → Router B → Router C, before reaching the destination subnet.

Routing decision principles

Routers apply the longest‑prefix match rule: the entry with the most specific subnet mask (longest mask) is preferred. After the prefix match, routers evaluate path‑cost metrics such as bandwidth, administrative distance, and metric values, considering both forward and reverse traffic.

Hop‑by‑hop lookup

Routing lookups are performed hop‑by‑hop; each router along the path must have routing information for the destination network and informs the packet of the next hop.

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