Ethernet and IP

A hierarchy

Ethernet and IP are part of a hierarchy of interchangeable technologies in the seven-layer OSI stack. While most of the networks that broadcast engineers are likely to encounter today are Ethernet and IP, it is important to understand that, in the early days, Ethernet was just one of a large number of competing Layer 2 technologies. Other Data Link Layer technologies include ATM, Token Ring and ARCNET.

While Layer 2 does a great job of organizing data into frames and passing them on to a physical network, it is not capable of allowing network architects to group computers into logical networks and allowing messages from those computers to be sent (or routed) to a much larger campus or even worldwide network. This is where Layer 3 comes in. IP operating at Layer 3 organizes data into datagrams, which can be sent across any Layer 2 networking technology. IP datagrams are the same size and the same format, regardless of whether these packets are sent across Ethernet, Token Ring or some other network. In fact, you might be surprised to learn that IP packets from your computer may first travel over Ethernet, then over a SONET for long-distance transmission, and then be put back into Ethernet for delivery on a local network at the destination.

IP is not the only Layer 3 protocol in common use today. There are a number of other critical protocols that operate at this layer, many of which have to do with configuring and maintaining networks. Internet Control Message Protocol (ICMP) and Open Shortest Path First (OSPF) are two examples of this.

In summary, Ethernet and IP are part of a hierarchy. IP packets are carried in Ethernet frames. IP packets that travel over long distances are likely to be carried in the payload portion of SONET frames. Furthermore, when you see the notation TCP/IP, remember that TCP is also part of this hierarchy. It is highly likely that every time you see this notation used in reference to traffic on a LAN, remember that you are actually talking about TCP over IP over Ethernet.