Ethernet Delivers the Internet
Most home-office or small-office situations bring the Internet into the local network through an Ethernet connection, leading to a basic router, interfaced with a DSL modem or a cable modem.
In order to understand the way Ethernet brings us the Internet, we have to take a little diversion into the concept of messages within messages.
Let me call your attention to a common, ordinary envelope. The outside of the envelope contains destination address and source address after the usual pattern that has been established for many, many years. In this case, the envelope is addressed to the Smith family, of 140 East Lilac, in Sunol, California. And, of course, the source address is placed in the upper left-hand corner according to well-established conventions.
It turns out that when a postman delivers this envelope to the Smith family according to this postal address, he’s performing a function that’s pretty similar to the way Ethernet delivers frames using Ethernet addresses.
Our example will get a little more interesting as we open up the envelope and see what’s inside. Of course, opening the envelope and looking inside is something that the postman will generally not do. He’s only concerned with the address on the outside of the envelope.
So, opening up the envelope, we can find out what’s inside. And that reveals that there’s another package inside the first one; another envelope inside the envelope, representing a message inside a message. This time the addressing system is still present, but it’s a little more intimate. It just says, “To Jessica, from Brian.”
Let’s find out what’s inside by opening up this inner envelope.
We find there a CD with a name on it and a description of its
contents on the outside. Opening up the CD jewel case will reveal that
farther inside is a CD-ROM with additional information on what it
contains. Of course, if we insert that CD into a CD player, we’ll get
some kind of an index of the songs on it.
As it turns out, we know something that the postman does not know, and that is that Jessica no longer lives here. So, let’s close up the package and insert it back into the inner envelope with the personal address identifying Jessica and Brian, and we’ll need to make a new outer envelope to address it to Jessica’s new, real address. The old envelope is simply discarded.
....There: a moment later, after addressing the new envelope to Jessica’s new address, we’ll re-insert the inner package inside the new envelope, and send it on its way to Jessica’s new address; a message inside a message.
In the field of computer networking and Internet working, this concept of placing one package inside another is extended to placing one message inside another. And it is commonplace. It’s been going on for a long time, and this practice has become well-known under the name of “Message Encapsulation.”