|Ethernet Delivers the Internet
Think back now to our first movie in our Ethernet series, the Ethernet
Tutorial. You’ll remember that that tutorial revealed that a lot of the
design for Ethernet is based on the behavior of telegraph operators.
Remember that as telegraph operations matured, the format for messages
became standardized. As telegraph operators sent a message, the first
part of each message was a description of the destination telegraph
office: the destination address to receive the telegram. After the
destination address, the telegram included the source address. In
essence, each telegram started with a preamble that said something like,
“Telegram to Salt Lake City, from Denver”. After those sections of a
telegram identifying the receiving station, and the sending station, the
main body of the text was transmitted.
Take a closer look at this telegram text.
You’ll see it contains a street address. This is a message within the
telegram: a message within the message. And this additional addressing
is necessary because telegraph addresses didn’t go everywhere; every
home did not have a telegraph receiver. A telegraph operator could
address the city and get to an area, but ultimate delivery of the
telegram contents to a person’s home required an additional address. A
boy on a bicycle would sometimes carry a hard-copy printout of a
telegram to a person’s house using the addressing information contained
inside the telegram.