Network layers

One way to look at layering in a network is as "an isolation of concerns."

No networking information would be complete without discussing the fact that IP networks (and SNA networks, too) are implemented as layers.

Figure 1. TCP/IP-based layered network

Each layer has certain capabilities (in the form of protocols) that it is required to uphold. For example, the IP layer does not include reliability of delivery, while the TCP layer does not concern itself with routing details.

Note: Isolation of concerns (layering) of protocols is not as foreign as it first appears.

For example, when you write a check, you conform to various requirements of the check writing protocol: you must include the amount, the payee, the date and a signature on each check. When it comes to delivery of that check, you put it in an envelope and follow the postal mail protocol to ensure you have a destination address, zip code, return address and, of course, correct postage.

The postal service doesn't care what protocol (the check writing protocol, in this case) is being encapsulated within the envelope. The content (payload) of a letter is not the concern of the mail protocol.