How Does a Router Work? Understanding Routing Basics and Path Selection

What Is a Router?

A router, also called a gateway device, connects multiple logically separate networks or subnets. It determines network addresses and selects IP paths, enabling flexible inter‑network connections and allowing multiple computers to share network access, including wireless connectivity.

Routing Table

Like a switch’s MAC table, a router maintains a routing table that records routes to each network node, including the data source, routing entries, and the next hop.

The routing table acts as the router’s navigation map; the router follows the entry that matches a packet’s destination IP. The table updates periodically and when network topology changes.

Packet Forwarding Process

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

Direct, Static, and Dynamic Routing

Direct (direct neighbor) : the router discovers neighboring networks automatically.

Static routing : administrators manually add routes; suitable for small networks but costly to maintain as the network grows.

Dynamic routing : routers learn routes automatically; large networks often combine dynamic and static routes.

Three‑Layer Addressing

Layer‑2 addressing uses MAC addresses for switching. Layer‑3 addressing uses IP addresses for routing.

When a host sends data, it first checks whether the destination is in the same subnet. If it is, the switch handles Layer‑2 forwarding. If not, the host forwards the packet to its router, which looks up the routing table and forwards to the next hop.

Routing Principles

Routers use the longest‑prefix match rule: the route with the most specific (longest) subnet mask is preferred.

After the prefix match, routers consider path‑cost metrics such as bandwidth, administrative distance, and other measures to select an efficient path for both forward and reverse traffic.

Routing lookup proceeds hop‑by‑hop; each router along the path provides the next‑hop information until the packet reaches its destination network.

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