9.9 The Physical Layer and Ethernet
One of the key points to understand about the internet is that it’s a software network. Nothing we’ve discussed so far is hardware specific, and indeed, one reason for the internet’s success is that it works on almost any kind of computer, operating system, and physical network. However, if you actually want to talk to another computer, you still have to put a network layer on top of some kind of hardware. That interface is the physical layer.
In this book, we’ll look at the most common kind of physical layer: an Ethernet network. The IEEE 802 family of standards documentation defines many different kinds of Ethernet networks, from wired to wireless, but they all have a few things in common:
- All devices on an Ethernet network have a Media Access Control (MAC) address, sometimes called a hardware address. This address is independent of a host’s IP address, and it is unique to the host’s Ethernet network (but not necessarily a larger software network such as the internet). A sample MAC address is 10:78:d2:eb:76:97.
- Devices on an Ethernet network send messages in frames, which are wrappers around the data sent. A frame contains the origin and destination MAC addresses.
Ethernet doesn’t really attempt to go beyond hardware on a single network. For example, if you have two different Ethernet networks with one host attached to both networks (and two different network interface devices), you can’t directly transmit a frame from one Ethernet network to the other unless you set up an Ethernet bridge. And this is where higher network layers (such as the internet layer) come in. By convention, each Ethernet network is also usually an internet subnet. Even though a frame can’t leave one physical network, a router can take the data out of a frame, repackage it, and send it to a host on a different physical network, which is exactly what happens on the internet.