How Does a Network Router Work?

Routing

Networks are designed to allow computers to talk to each other. This facilitates file sharing, email and remote access. This is easily accomplished when all the computers are on the same physical network. But it becomes a bit more challenging when the computers are spread out around the world. Routers are used to send TCP/IP packets from one network to another. Routers learn what computers are attached to what port and use that information to send packets to their destination.

ARP

Routers use a protocol known as Address Resolution Protocol (ARP) to learn about attached computers. An ARP frame is sent as a broadcast frame. Unlike other frames that are sent to a specific destination address a broadcast frame is sent to all computers on the network and must be responded to. The computers respond with its IP address and its MAC address. The IP address is assigned either by the user or by a DCHP server, the MAC address is a unique address assigned to the network card when it is manufactured. The router builds a table with this information and the port the response was received on. The router uses this table to determine if it knows the destination address when it receives a packet. If the destination address is in the routing table the router forwards the packet to the port in the table. If the address is not found in the routing table it is sent to the default gateway.

Useful tools

There are two networking tools that are useful when troubleshooting a routed network. The first is the Ping command. A ping is a special frame that is sent to a destination computer, if the computer is attached to the network and operational it will respond with a ping response. The ping and the response frame contain the IP address and the MAC address. If the routing table is out of date it might route the frames to the wrong MAC address.

The second tool is trace route. Traceroute will detail the steps a packet takes to get to it's destination, it lists each router's IP address and the time it's taken to get to the router. This tool is very helpful when diagnosing connection failures or network slowness.