What a Router Actually Does

Switches connect devices within one network, but to reach a different network — like a website on the internet — you need a router. A router's job is to move traffic between separate networks, choosing the best path toward each destination. Routers are what stitch the world's networks into the internet. This lesson explains what a router really does and how it differs from a switch.

A router works at the network layer (Layer 3 of the OSI model), where data is addressed with IP addresses (Internet Protocol addresses). While a switch uses MAC addresses to move frames inside a LAN, a router uses IP addresses to forward packets between networks. This is why a router is a Layer 3 device and a switch is a Layer 2 device.

Each router interface connects to a different network or subnet. A home router, for example, has one interface facing your internal home network and another facing the internet through your ISP. The router sits at the boundary between networks and decides how to pass traffic from one to the other, translating between the separate address spaces.

When a packet arrives, the router reads its destination IP address and consults its routing table — a list of known networks and the best next step toward each. It then forwards the packet out the appropriate interface toward its destination. If no specific route matches, the router uses a default route to send the packet onward, typically toward the internet.

The routing table is the router's map. Each entry pairs a destination network with the next hop (the next router or interface to send traffic to) needed to reach it. Routers build these tables from directly connected networks, manually entered static routes, and routes learned automatically through dynamic routing protocols — topics you will explore in the routing course.

To devices on a LAN, the router appears as the default gateway — the address they send traffic to when the destination is outside their own subnet. When your computer wants to reach a distant website, it hands the packet to its default gateway (the router), which then forwards it toward the internet. Without a default gateway, a device cannot reach other networks.

An important behavior: routers do not forward broadcasts between networks. This means each router interface bounds a broadcast domain, helping keep broadcast traffic contained to one network. By stopping broadcasts at the boundary, routers prevent broadcast storms from spreading across the whole internet and keep each network's local chatter local.

Keep the two devices distinct:

• Switch — Layer 2, uses MAC addresses, moves frames within one network.
• Router — Layer 3, uses IP addresses, moves packets between networks.

A switch builds the local network; a router connects local networks to each other and to the internet. Many home devices combine both roles in one box.

Routers often do more than route. Home and small-office routers also perform NAT (Network Address Translation) to let many private devices share one public IP, run a DHCP server to hand out addresses, and include basic firewall features. These added roles are why a single home "router" box is really several network functions combined for convenience.

The line between switches and routers can blur. A Layer 3 switch is a fast switch that also performs routing between VLANs in hardware, combining a switch's speed with a router's ability to move traffic between networks. Large networks use Layer 3 switches internally to route between many VLANs quickly, while a traditional router still connects the site to the internet. Recognize the Layer 3 switch as a hybrid device.

Without routers, networks would be isolated islands unable to reach one another. Routers make the internet possible by interconnecting countless networks and finding paths across them. For the exam, anchor these facts: a router is a Layer 3 device using IP addresses to forward packets between networks via a routing table, it acts as the default gateway, and it stops broadcasts at network boundaries.

Test your router knowledge.

You learned that a router is a Layer 3 device that moves packets between separate networks using IP addresses. It reads each packet's destination IP, consults its routing table for the best next hop, and forwards accordingly, using a default route when no specific match exists. To LAN devices it is the default gateway. Routers do not forward broadcasts, bounding broadcast domains. They often add NAT, DHCP, and firewall roles, making the internet possible by linking networks together.