Routing is the process by which data packets move from one node (machine or device) to another on a computer network until the packets reach the final destination.Yagi Studio / Getty Images
Understanding Network Routing
You can think of the way data is routed as similar to how you might get to a far-off place in your city using a series of bus trips. The entire bus system, including all the stops, is like the network, and the stops are like the nodes. As a bus rider who must make several transfers to get where you're going, you're like the data that travels between each node until it reaches its final destination.
When data is transferred from one device to another on an Internet Protocol (IP) network, it's is broken down into smaller units called packets. In addition to the actual data, each packet includes a header that contains information to help it get to its destination, similar to the physical address information you might find on a mailed envelope. But, instead of physical addresses, the header information includes:The IP addresses of the source and destination nodes.Packet numbers that help reassemble the packets in the correct order when the packets reach the destination.Other useful technical information.
How Routing Works
Consider a scenario in which Li sends an email message from his computer in China to Jo's machine in New York. Transmission Control Protocol (TCP) and other protocols work with the data on Li's machine, and then it's sent to the IP module, where the data packets are bundled into IP packets and sent over the network. To reach the destination on the other side of the world, the data packets must pass through many routers. The work these routers do is called routing.
The IP and TCP protocols work together to ensure transmissions are reliable, meaning that no data packets are lost, data packets are in order, and there's no unreasonable transmission delay. In some services, TCP is replaced with Unified Datagram Packet (UDP), which doesn't ensure reliability, but instead sends packets over. Some Voice over Internet Protocol (VoIP) systems use UDP for calls because lost packets don't affect call quality.
Each of the intermediate routers reads the destination IP address of each received packet. Based on this information, the router sends the packets in the appropriate direction. Each router has a routing table where information about neighboring routers (nodes) is stored. This information includes the cost (in terms of network requirements and resources) of forwarding a packet in the direction of that neighboring node. Information from this table is used to decide the most efficient node to use or the best route on which to send the data packets. Each packet can be sent in a different direction, but all eventually get routed to the same destination machine.
On reaching Jo's machine, the packets are consumed by the machine, where the IP module reassembles the packets and sends the resulting data to the TCP service for further processing.