The following list depicts the process shown in the figure:
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1. |
Someone has written a letter and addressed it to me at an
address in Mason, Ohio. The person either brings the letter to the post office
in Cincinnati or drops it in a mailbox in Cincinnati. Either way, the letter
ends up in the post office in Cincy, and a decision needs to be
made.
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2. |
Either a postal worker or more likely a mail-sorting machine
sorts the letter. The sorter puts the letter shown in the figure into a pile of
letters that will be put into a truck and driven to Mason. The letter is sent to
the Mason post office.
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3. |
At the Mason post office, the letter is sorted again.
However, in this case, the sorter in Mason looks at the whole address, including
the street address. The letter is put on the truck that passes by 1010 Elm
street in Mason each day. That way, the driver can stop at that address and
leave the letter in the mailbox.
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IP routing works somewhat like the way the USPS forwards
letters. For instance, the IP routing process relies on the fact that all IP
addresses on the same physical networksuch as all IP addresses on one Ethernet
LANhave a portion of their IP addresses in common. This part of the address that
is the same on all IP addresses on that physical network works just like a zip
code. Just as zip codes identify a bunch of postal addresses that are in the
same general area, all the IP addresses on the same Ethernet are in the same
general area, so routing can take advantage of that fact.
Examples definitely help for this idea. Consider Figure 10-6, which shows a simple
internetwork with two routers and three IP networks.
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