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Because a socket is the equivalent of a phone extension, the program
that picks up the call is the equivalent of the person within the
house or company that you want to talk to (see Figure 1.4). The
program can do things for you once you "talk"-either give you
information or kick off a process that you need accomplished. Some
requests can be a combination of both: When you search the Internet,
you ask the program on the other end to do a search, and you ask for
the answer to your search.
[01-04t.jpg]
Figure 1.4 A socket is the equivalent of a phone extension.
Just like people, some programs (also called services because they're
server programs) on the other side of the line speak only one
language. For example, standard Web servers speak only TCP/IP.
However, as previously mentioned, Microsoft file and print sharing
services can run on just about anything: TCP/IP, IPX/SPX, and NetBEUI.
As with teaching a person a new language, it takes a bunch of effort
to program multiple-protocol support into a service, and unless
there's payback involved, most programs just pick one protocol and
stick with it. The most popular protocol for modern programs? You
guessed it: TCP/IP.
What's in a Name?
You've probably asked yourself in one of the preceding paragraphs,
"How does www.co.chatham.ga.us get translated into 167.195.160.9?"
Furthermore, why use names at all? People can deal with phone numbers,
why not just use the IP number? These are good questions. The answer
to the latter is that just because people can deal with a number
doesn't mean that they prefer to use a number. Which would you rather
remember, 1-800-NETWORK or 1-800-638-9675? Obviously, most people
prefer to remember a name. Actually, names are the better thing to use
when networking, because numerical addresses can change during a
reconfiguration or a move, whereas symbolic names typically stay the
same.
Name-to-address translation (also known as name lookup or name
resolution) occurs via name services. Very similar to the speed dial
button on your phone, name services are the networking equivalent of
an electronic phone book. They're actually a lot cooler than your
speed dial: For example, suppose you could say "Mom" to make your
phone dial your mother.
DNS
Name services run as a service on any given name server; that is, a
specific program runs on a name server that hands out an address when
you give it a name. Like your speed dial buttons, you must program in
a name entry; entering the correct number for a given name is
important.
In particular, TCP/IP name services, although powerful and able to
handle millions and millions of names, isn't exactly plug-and-play.
The DNS (Domain Name Service) that you use when surfing the Web works
pretty automatically for you once it's configured correctly, and it
will translate www.co.chatham.ga.us to 167.195.160.9. However, you'll
need to know the exact number of your DNS server. Unlike telephone
information, DNS servers all have different addresses; verifying that
a workstation's DNS server is correct can be an important
troubleshooting step (see Figure 1.5).
[01-05t.jpg]
Figure 1.5 Name resolution on DNS.
Note that most smaller sites that use TCP/IP usually don't have DNS
set up. Instead, each workstation has a local (hard drive) "hosts"
file that lists the addresses and host names the workstation needs to
get to. (Think of this as your personal phone book rather than the
corporate directory.) As you can imagine, this gets hard to manage
when you have more than a handful of workstations, unless the
addresses of the servers never change. As sites grow, or as they get
connected to the Internet, DNS servers are added. Can you imagine how
big a single file with all the servers on the Internet would be?
Fortunately, each DNS server for a given DNS zone is only responsible
for its own information.
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A DNS zone (its scope of responsibility for naming) can be huge-for
example, .com has millions of subzones (yahoo.com, jotto.com, and
so on); on the other hand, it can be small-for example, feldman.org
lists only one host (www.feldman.org) and no subzones.
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With DNS servers getting easier to manage and being a mandatory
component of Internet access, you can expect to see more of them in
smaller shops as time goes on. It's worth mentioning that each DNS
server is responsible for only its own zone, so if you can't get to
one particular address (say, yahoo.com) but can get to another (say,
jotto.com), it may be that the name server responsible for that zone
is down. On the Internet at large, this rarely happens, because the
DNS organizers require back-up DNS servers for a zone. DNS problems
are more likely to happen within a smaller organization's intranet,
particularly when all the eggs for that organization are in one
basket.
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Remember that you can still dial a number yourself when your speed
dial buttons are broken. Similarly, if you cannot get to something
on your network, try getting to it by number. For example, rather
than going to http://www.co.chatham.ga.us, you could try
http://167.195.160.6. If this works, you know there's something up
with name services.
______________________________________________________________
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