Tài liệu Chapter-18-Connecting to the Internet pptx

2Apr il 2003, 17:00:47 The Complete FreeBSD (isp.mm), page 315
18
Connecting to the
Internet
In this chapter:
• The physical
connection
• Establishing yourself
on the Internet
• Choosing an Internet
Ser vice Provider
• Who’sthat ISP?
• Making the
connection
In this chapter:
• The physical
connection
• Establishing yourself
on the Internet
• Choosing an Internet
Ser vice Provider
• Who’sthat ISP?
• Making the
connection
To implement the reference network shown in the previous chapter,weneed to do a lot of
things that interface with the outside world. Theycan takesome time, so we should look
at them first:
• What kind of physical connection should we use? We’llconsider that in the next
section.
• We may want to register a domain.Manypeople don’t, but I strongly recommend it.
Find out about that on page 317.

• We may also want to register a network.Inour example, we have used the network
223.147.37.0.Inreal life, we can’tchoose our own network: we takewhat is given
to us. We’lllook at this on page 318.
• We need to find an Internet Service Provider.We’ll look at what that entails on page
319.
The physical connection
Just twoorthree years ago, the way to connect to the outside world was simple: a phone
line. Since then, things have changed quite a bit, and you may have quite a choice:
• Analogue telephone line connections are still the most common way of connecting
small networks in most countries, but their bandwidth is limited to about 7 kB/s at
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The physical connection 316
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best. You can run PPP or SLIP overthis kind of line, though nowadays most ISPs
support only PPP.
• ISDN stands for Integrated Systems Digital Network.It’sthe new, better,washes-
whiter telephone system that is replacing POTS (Plain Old Telephone Service)in
some countries, notably in Europe. FreeBSD supports ISDN with the isdn4bsd
driver. Wewon’tlook at ISDN further in this book.
• Leased lines form the backbone of the Internet. They’re invariably more expensive
than dialup lines, but theycan provide quite high speeds—in the USA, a T1 line will
give you 1,536 kbps, and in the rest of the world an E1 will give you 2,048 kbps.
Leased lines are becoming less interesting, and we won’tlook at them in more detail
in this book.
• Cable modems use existing cable TV networks to deliverahigh speed connection, up
to several megabits per second. Theyuse the cable as a broadcast medium, rather like
an Ethernet, and suffer from the same load problems: you share the speed with the
other users of the cable. There are also some security issues to consider,but if you
have a cable service in your area, you’ll probably find it superior to telephones. The
cable modem is effectively a bridge between the cable and an Ethernet. From the

FreeBSD point of view, the cable modem looks likejust another Ethernet device.
• DSL (Digital Subscriber Line)isthe telephone companies’ reaction to cable modems.
Until recently,the L stood for Loop,not Line.Aloop is the telco term for the pair of wires
between the exchange (or Central Office)and the subscriber premises.
There are a number of variants on DSL: ADSL (Asynchronous Digital Subscriber
Line)has different speeds for the uplink and the downlink, while SDSL (Symmetric
Digital Subscriber Line)and HDSL (High-speed Digital Subscriber Line)hav e the
same speed in each direction. Speeds and capabilities differ widely from one location
to another.Bymodifying the way theytransmit data overnormal phone wires,
including the use of special modems, ADSL can get speeds of up to 6 Mb/s
downstream (towards the end user), and about 640 kbps upstream. HDSL has similar
speeds, but the speed is the same in each direction. In contrast to cable modems, you
don’thav e to share this bandwidth with anybody.Technical considerations limit the
loop length to about four miles, so eveninbig cities you may not be able to get it.
ManyDSL services are plagued by technical problems. There are a number of
different ways to connect to a DSL service, but most of them involveaconversion to
Ethernet.
• In some parts of the world, satellite connections are a viable alternative.These
usually use a telephone line for outgoing data and a satellite receiverfor incoming
data. Pricing varies from very cheap to quite expensive,but if you can’tget cable or
DSL, this might be your only choice.
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Establishing yourself on the Internet
The first thing you need to decide is the extent of your presence on the Net. There are
various possibilities:
• Youcould get a dialup service where you use your computer just to connect to the
ISP,and perform network functions such as reading mail and news on the ISP’s
machine (a shell account). It’salot faster to perform these functions on your own

machine, and you have all the software you need to do so, so this option isn’tvery
desirable. This option is becoming increasingly uncommon.
• Youcould perform all the functions on your machine, but using names and addresses
assigned to you by the ISP.
• Youcould perform all the functions on your machine, using addresses assigned to
you by the ISP,but you would use your own domain name.
• Youget your own address space and use your own domain name.
Does it matter? That’sfor you to decide. It’scertainly a very good idea to have your own
domain name. As time goes on, your email address will become more and more
important. If you get a mail address like ,and Flybynight goes
broke, or you decide to change to a different ISP,your mail address is gone, and you have
to explain that to everybody who might want to contact you. If, on the other hand, your
name is Jerry Dunham, and you register a domain dunham.org,you can assign yourself
anymail address in that domain.
But howdoyou go about it? One way would be to pay your ISP to do it for you. You
don’tneed to do that: it’seasy enough to do yourself on the World-Wide Web.You must
be connected to the Internet to perform these steps. This implies that you should first
connect using your ISP’sdomain name, then establish your domain name, and change to
that domain.
Whichdomain name?
We’llcontinue to assume that your name is Jerry Dunham. If you live in, say,Austin,
Te xas, you have a number of domain names you can choose from: dunham.org,
dunham.com, dunham.net,oreven dunham.tx.us if you want to use the geographical
domain.
If you live in, say,Capetown, people will probably suggest that you get the domain
dunham.za,the geographical domain for South Africa. The problem with that is that
you are limiting yourself to that country.Ifyou move to, say,Holland, you would have to
change to dunham.nl—a situation only fractionally better than being bound to an ISP.
The same considerations apply to dunham.tx.us,ofcourse.
Your choice of domain name also affects the way you apply.Inthe following sections, I

assume you takemyadvice and apply for an organizational rather than a geographical
domain.
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Preparing for registration
Once upon a time, registration was handled by InterNIC, a professional body.Since then
it has been delegated to commercial companies, and the quality of service has suffered
correspondingly: theydon’tevenappear to knowthe technical terms. Forexample, you
may find them referring to a domain name as a ‘‘Web Address.’’ Things are still
deteriorating at the time of writing: additional companies are being allowed to register
domain names, and the field seems to attract a lot of cowboys.
Registering a domain name
The only prerequisites for registering a domain name are:
• The name must be available, though there are some legalimplications that suggest
that, though you might be able to register a domain such as microsoft.edu,itmight
not be good for you if you do. In fact, microsoft.edu wasonce registered to the
BISPL business school in Hyderabad, India, presumably not in agreement with
Microsoft.
• Youmust be able to specify twoname servers for it—see Chapter 21 for further
details about name servers.
First, check that the name is available:
$ whois dunham.org
No match for "DUNHAM.ORG".
The InterNIC Registration Services Host contains ONLY Internet Information
(Networks, ASN’s, Domains, and POC’s).
Please use the whois server at nic.ddn.mil for MILNET Information.
Next, try to find a reputable registrar.Immediately after the transfer of registrars from
InterNIC, the only companytooffer this service was Network Solutions, but nowthere
are many. I do not recommend Network Solutions: they’re expensive and incompetent.

If, as I recommend, you set up your mail server to refuse mail from servers without
reverse mapping, you will not be able to communicate with them, since theydonot have
reverse DNS on their mail servers, and theyuse unregistered names for them. Judge for
yourself what this says about their technical competence.
One registrar that manyFreeBSD people use is Gandi ( ), which is
slightly associated with the FreeBSD project. So far nobody has found anything negative
to say about them. UnlikeNetwork Solutions, their web pages are also relatively simple
to understand.
Getting IP addresses
Once upon a time, it was possible to get IP addresses from InterNIC, but this practice is
nowrestricted to large allocations for ISPs. Instead, get the addresses from your ISP.
Routing considerations makeitimpractical to move IPaddresses from one place to
another.Ifyou move a long distance, you should expect to change your IP addresses in
the same way as you would change your telephone number.
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Choosing an Internet Service Provider
In most cases, you will get your connection to the Internet from an Internet Service
Provider,orISP.Asthe name suggests, an ISP will supply the means for you to connect
your system or your local network to the Internet. Theywill probably also supply other
services: most ISPs can’tliv e on Internet connections alone.
In this chapter we’ll look at the things you need to knowabout ISPs, and howtoget the
best deal. We’llconcentrate on what is still the most common setup, PPP overadialup
line with a V.90 modem (56 kbps), which will give you a peak data transfer rate of about
7kB/s.
Who’sthat ISP?
As the Internet, and in particular the number of dialup connections, explodes, a large
number of people have had the idea to become involved. In the early days of public
Internet access, manyISPs were small companies run by very technical people who have

seen a market opportunity and have grabbed it. Other ISPs were small companies run by
not-so technical people who have jumped on the bandwagon. Still other ISPs are run by
large companies, in particular the cable TV companies and the telephone companies.
Which is for you? Howcan you tell to which category an ISP belongs? Do you care?
You should care, of course. Let’sconsider what you want from an ISP,and what the ISP
wants. You want a low-cost, high-reliability,high speed connection to the Internet. You
may also want technical advice and value-added services such as DNS (see Chapter 21)
and web pages.
The main priority of a small ISP (or anyother ISP,for that matter) is to get a good night’s
sleep. Next, he wants to ensure the minimum number of nuisance customers. After that,
he wants to ensure that he doesn’tgoout of business. Only then is he interested in the
same things that you are.
In the last fewyears, a large number of ISPs have gone out of business, and manymore
have merged with other companies. In particular,large companies frequently bought out
small techie ISPs and then ran them into the ground with their incompetence. Fora
humorous viewofthis phenomenon, see the ‘‘User Friendly’’cartoon series starting at
/>Questions to ask an ISP
So howdoyou choose an ISP? Don’tforget the value of word-of-mouth—it’sthe most
common way to find an ISP.Ifyou knowsomebody very technical, preferably a
FreeBSD user,who is already connected, ask him—he’ll certainly be able to tell you
about his ISP.Otherwise, a lot depends on your leveloftechnical understanding. It’s
easy to knowmore about the technical aspects of the Internet than your ISP,but it doesn’t
often help getting good service. Here are a fewquestions to ask anyprospective ISP:
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