Internet Protocol (IP) Routing
Internet Protocol (IP) addresses are similar to postal mailing addresses that define where data should be “delivered” on the internet. Every host on the internet has an IP address. The process of delivering data packets to every host on the internet is called “Routing”.
A basic IP routing process works like this:
1. Your local computer wants to send a data packet to a destination address. If the destination is the same as the local computer the computer simply delivers the data packet to itself. (Yes – it’s odd to mail yourself at your same address, but computers do this all the time).
2. If the destination is on the same network or subnet as your local computer, your local computer sends the packet directly to the destination computer. This is like walking a birthday card over to your neighbor’s house without handing it to the postal service for delivery.
3. If the data packet destination is on a network outside of your local network, your computer will send that data packet to the router on your local network for delivery. Your local router could be as simple as a Wi-Fi router provided by your Internet Service Provider or a larger router located in your office building. This routing step is like giving the birthday card to the post office for delivery to a destination outside your neighborhood.
4. The router that connects your local network to the greater internet doesn’t really know how to deliver the data packet to the destination. Your local router sends any traffic destined for the larger internet to a much larger router located at your Internet Service Provider. These large ISP routers know about some internet destinations, but not all possible destinations. Think of this step as the birthday card arriving at a regional postal hub for sorting.
5. Routers that connect very large networks like Charter, Comcast, Verizon, CenturyLink and others know about many Internet paths, but not all of them. There are too many possible networks with multiple paths to each network for any one router to know them all. Large Internet routers exchange network and path information via a protocol called Border Gateway Protocol (BGP). This is how routers discover other routers and the networks they service. This mechanism is a bit like our birthday card being sent to the postal facility of another country. Our postal service facility may not know all the local roads to the destination, but the other postal facility does.
6. Once your ISP’s router learns the path or hop to the next router the data packet is sent to that router. Sometimes many hops are needed to get to the final destination router. Think multiple mail sorting facilities until the birthday card arrives in the destination mailbox.
This is a highly simplified description of the process. The beauty of this system is that there is quite a lot of redundancy built into the internet. Most ISPs have many routers and many paths to each other. The failure of a router or path usually will not prevent reliable delivery of data packets to their destinations.