Ethernet and Internet

Ethernet and Internet — Rob Russell of Auckland, New Zealand <[email protected]> pointed out what seemed to be a contradiction in the second installment of the Hey, I’m Talking To You article (see NetBITS-002) about how machines find each other over an Ethernet network and over the Internet. The article stated the Internet doesn’t use Ethernet, because Ethernet LANs – segments in which all the machines "see" each other – only work over short distances. But if you hadn’t read part 1, last week’s sequel seemingly implied TCP/IP couldn’t run over Ethernet.

The confusion probably stems from writing about physical protocols (like Ethernet) and data protocols (like TCP/IP) at the same time. Data you send, such as a Web page request, is encoded into a set of TCP packets with IP addresses on them. Once the packets leave the application that assembles them on a particular machine, the physical protocols take over to move the packets along the machine’s physical network connection. One Ethernet device talks to another to pass along the TCP packets; if the packets are bound for an outside network, they may go over a serial line that uses a different physical protocol. So regardless of whether you’re sending data over a local Ethernet or between routers using leased telephone company lines, the data format is still TCP using IP addressing. The medium, in this case, is not the message, just the carrier. [GF]