The Post-Office analogy

(This article is part of a series network-basics)

A nice way to illustrate the workings of an IP-network is to compare it’s addressing and routing to a traditional postal service.

In a computer network the material that needs to be transported is chopped into “packets”. Each packet has address information, the actual payload (the stuff that the user wants to transport) and then some. A packet could be compared to a single postcard or a letter send via post.

Here is an address on a letter:

John Smith
Long Street 1001
00500 Example City

It is hierarchical. The postal service would use the address starting from the bottom, from most generic information. They would first deliver this letter to the right city by their scanners reading the postal code’s first few digits. Knowing which direction the letter is going, they would put it to a delivery car that takes it one node closer to the destination city or area. When it reaches the right city’s main office, they will then deliver the letter to the right Post Office by reading the rest of the postal code. Finally the mail man operating at this area would deliver the letter to the correct mail box.

At this point the Postal Service (the Network) has done its job.

Quite similarly the IP network will read portions of the destination address and route the packet closer to the recipient. When the sending host has given the packet to the network to be carried, each node (router) will look at its internal instructions (routing table) to know to which direction (next-hop-address) the packet should be forwarded.

As a result of many routers doing this, the packet will be forwarded, one hop at the time, to the router that is “closest” to the recipient. This router then will pass the packet to the destination host, just like the mail man carried the letter from the closest Post Office to the customer’s mail box.

Sources

http://www.informit.com/articles/article.aspx?p=130834&seqNum=3
http://www.cs.gmu.edu/cne/itcore/internet/tcpip/tcpip.html
http://en.wikipedia.org/wiki/Internet_Protocol#IP_addressing_and_routing
http://www.microshell.com/sysadmin/networking/laymans-understanding-of-networking-udptcpip/

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