Wednesday, 24 July 2013

computer network connectivity

computer network connectivity



You have a computer, and you know that there is useful information on this computer, but not very much. You also know that other people, millions of other people also have computers, and that their computers will also have useful information.

Now, you can assume that these other people, and these other computers may very likely have lots of information on them that would be of interest to you. The only problem is how to access all this useful information that may be on other people's computers.

The computers themselves can communicate with each other, easily, through ports, using the different protocols that have been designed, but that doesn't really help you. You can't understand the streams of binary data that the computers exchange between themselves.

You need some way for your computer to interpret the information that it can receive from the other computers in some way that you can use it.



The programs that the computers use to translate the data that they exchange into a form that is useful to you are call services. These services allow you to view web pages, exchange e-mail, chat, and interact in remote computers in many other different ways.

Your computer, the local computer uses programs called clients to interpret the information that you receive. The other computers, the remote computers, use programs called servers to provide this information to your computer.


HTTP and The Web


When you say, 'the Internet,' what comes to mind for most people is, in fact, the World Wide Web. The World Wide Web, or just the Web, is not the Internet. Instead, it is a method of using the Internet to exchange information between computers. The Web uses http or hypertext transfer protocol and services known as web browsers and web servers to allow information in the form of web pages to be exchanged between local and remote computers.

On the local side, what you see is the web browser. Information from the remote computer is sent to your local computer using the http protocol. The web browser interprets that information and displays it on your local computer in the form of web pages.

The hypertext part of the http protocol refers to a non-linear method of presenting
information. Text is normally read in a linear fashion: word 2 follows word 1; sentence 3 follows sentence 2; paragraph 5 follows paragraph 4. The idea of hypertext allows information to be viewed in a non-linear way. This is the major difference between hypertext and the older, plain text methods of displaying information.

With hypertext, words and ideas can connect, not only with the words that directly surround them, but also with other words, ideas or images. Hypertext is not restricted to the Web. Most full-featured word processors will allow you to create locally stored pages in web or http format. These pages are read using your web browser and act as would any other web page, only they are stored on your local computer, not a remote computer.

On your local computer, you use a client program called a web browser. Contrary to what you might have been lead to believe, there are actually a number of web browsers available for both Windows and Linux. These include Microsoft's Internet Explorer, Netscape Navigator, and the Mozilla Firefox browsers.

You can also create your own web page. The easiest way to do this is to use one of the common word processors, such as OpenOffice, Microsoft Word, or WordPerfect. These programs will allow you to produce simple web pages, combining text, hypertext and images.

Plenty of people have made useful, clever and innovative web pages using these simple tools.

But these pages aren't flashy. Flashy means frames and scripts and animations. It also means spending lots of money on a fancy web page design program. These programs allow you to create many interesting effects on your web page, but they are more complex to use than the word processors that you are probably already familiar with.

Once you have the pages designed, you'll need a computer to put them on, so that other people can view them. This is called web hosting.

The hosting computer will be running a web server. It is possible to run one of these servers from your own home, using your own computer, but there are several drawbacks, the primary one of these being persistence. Information stored on a web server is only available when that server is powered up, operating properly and has an open connection. So, if you want to
run a web server from your own bedroom, you have to leave your computer on all the time; you have to make sure that the web server program is operating properly all the time (this includes troubleshooting hardware problems, controlling viruses, worms and other attacks,
and dealing with the inevitable bugs and flaws within the program itself), and you have to keep a connection to the Internet open. This is why most people pay someone else to do all this.

A web hosting company will store your web page on their computer. A perfect web hosting company will have multiple, redundant servers and a regular backup policy, so that your service is not lost because of hardware problems, a support staff to keep the server running despite hacker attacks and program bugs, and a number of open connections to the Internet, so that all your have to do is design your web page, upload it to the hosting company's server, hang up the phone, turn off the computer, and go to sleep, and your web page will be available to the entire world.

It's also possible to find organizations that offer free web hosting. Some of these organizations are funded by paid advertising, which means that anyone who wants to view your web page will first have to view someone else's advertisement. But they don't have to buy anything, and you don't have to pay anything.


 E-Mail – POP and SMTP


The second most visible aspect of the Internet is probably e-mail. On your computer, you use
an e-mail client, which connects to a mail server. When you set up your e-mail account, you
are given a unique name in the form of user@domain. You are also asked to provide a
password to use to retrieve your e-mail.

The SMTP protocol, which is used to send e-mail, does not require a password. This may not
have been a fault when the protocol was designed, and the Internet was a small world
inhabited by like minded people, but now it has become a loophole which allows for
unauthorized use of mail servers and various other tricks, such as 'e-mail spoofing', in which
someone sends an e-mail that appears to come from another address. However, some mail
servers minimize this flaw by implementing an authentication step, in which you must prove
your identity before you can send an e-mail.

One important thing to remember is, despite being password protected, e-mail is not a way
to send secure information. Most POP clients and servers require that your password be
communicated – unencrypted – to your mail server. This doesn't mean than anyone who
receives an e-mail from you also receives your password; but it does mean that someone with
the right knowledge and tools can relatively easily 'sniff out' your password.


IRC (Internet relay chat)


IRC, or Internet relay chat, is where the unregulated nature of the Internet is most clearly
expressed. On IRC, anyone with anything to say gets a chance to say it.

You may be familiar with the chat rooms used by certain online services. IRC is just like a chat
room, only there are no rules, there are no standards, and – quite often – there are no
chaperones. You may find exactly what you are looking for on an IRC channel, or you just
may find something that you had rather you never knew existed.

All the rules that you've heard about chat rooms are applicable to IRC channels. Don't tell
anyone your real name. Don't give out your phone number, your address, or your bank
account numbers. But have fun!


FTP


FTP stands for file transfer protocol. As the name implies, it allows for files to be transferred
between a local and a remote computer. While it can be used for private file transfers, it is
more commonly associated with free, anonymous ftp servers which offer public access to
collections of files.

Anonymous ftp was once the means by which most computer users exchanged files over the
Internet. While many anonymous ftp servers are used to distribute files that are available
illegally(and are possibly infected with viruses), there are also many which are legally used to
distribute programs and files. Servers which offer anonymous ftp services can be found
through various means, including Internet search engines.
Most anonymous ftp servers now allow you to access their files using the ftp protocol through
a web browser.


Telnet and SSH


Telnet allows a local user to send a wide variety of commands to a remote computer. This
allows the local user to instruct the remote computer to perform functions and return data to
the local computer, almost as if you were sitting at a keyboard in front of the remote
computer. SSH, or secure shell is intended as a secure replacement for telnet.

Again, both Windows and Linux come with a basic, command line telnet client; to access it,
open a command prompt or terminal window and type: telnet.

To access a telnet server, you will need to have an account and password set up for you by
the administrator of the server, because the telnet program allows you to perform a large
number of actions, some of which could severely compromise the remote computer.
Telnet was used in the past to allow computer administrators to remotely control servers and
to provide user support from a distance.

Telnet can also be used for a number of other tasks, such as sending and receiving email and
viewing the source code for web pages (although telnet does fall under the heading of the
most difficult way to do these things). Telnet can be used to do many things that are illegal
and immoral, but there are also legitimate reasons for using it. You can use telnet to check
your email, and view, not just the subject line, but the first few lines of an email, which will
allow you to decide whether or not to delete the email without downloading the entire
message.


 DNS


When you want to call a friend on the phone, you need to know the correct phone number;
when you want to connect to a remote computer, you also need to know its number. You
may remember from previous lessons that, for computers on the Internet, this number is called
the IP address.

As numbers, these IP addresses are very easily managed by computers, but as humans, we
prefer to use what are called domain names. For example, to connect to the Hacker
Highschool web page, we type 'www.hackerhighschool.org' into the address bar of a web
browser. However, the web browser can't use this name to connect to the server that hosts
the Hacker Highschool web page – it must use the IP address. This means that your local
computer must have some means of translating domain names into IP addresses. If there
were only hundreds, or even thousands of computers on the Internet, then it might be possible
for you to have a simple table stored on your computer to use to look up these addresses, but,
not only are there are millions of computers on the Internet, the correlations between domain
names and IP addresses can change daily.

For this reason, DNS or Domain Name Service is used to translate domain names into IP
addresses. When you type the domain name www.domainname.com into your web browser,
your web browser contacts the DNS server chosen by your ISP. If that DNS server has
www.domainname.com in its database, then it will return the IP address to your computer,
allowing you to connect.

If your DNS server doesn't have www.domainname.com in its database, then it will send a
request to another DNS server, and it will keep sending requests to other DNS servers until it
finds the correct IP address, or it establishes that the domain name is invalid.



 DHCP


DHCP or Dynamic Host configuration Protocol allows for IP addresses to be dynamically
allocated within a network. The network is given a block of IP addresses for its use. When a
computer joins the network, it is assigned an IP address. When a computer leaves, its IP
address becomes available for use by another computer.

This is useful for large networks of computers, since it is not necessary for each computer to
have an individually assigned, static IP address. Instead, you use a DHCP server. When a new
computer connects to the network, the first thing that it does is request an IP address from the
DHCP server. Once it has been assigned an IP address, the computer then has access to all
the services of the network.


 Connections


Most computers connect to the Internet through a modem. Modems translate the digital
signals produced by computers into analog signals that can be transmitted across commonly
available telephone lines. Modem speeds are measured in baud or bits per second. Higher
baud rates are better, since they allow for faster transmission of data, but you must also
consider what you are planning to do. There are certain applications – such as telnetting into
MUDs – for which a twenty year old 300 baud modem would still be acceptable (provided
your typing speed wasn't so good), while high bandwidth applications such as streaming
video can often strain even the most powerful cable modems.


 ISPs


You don't just call up the Internet. You need to access a server that will connect your
computer to the Internet. The server does all the heavy work, like being on all the time. The
server is run by an ISP or Internet Service Provider.

An ISP has a point-of-presence on the Internet that is constant, and it has servers that run the
services you are going to use. Now, you can run these services on your own. For example, you
can run a mail server on your local computer, but it will require you to have your computer
powered up and connected to a network all the time, just waiting for those brief moments
when information has to be exchanged. An ISP, however, consolidates the efforts of a large
number of users, so the mail server is working all the time, instead of sitting around, doing
nothing. Additionally, an ISP's computers are going to use a high speed connection to
connect to a NAP or Network Access Point. These NAPs then interconnect with each other
through ultra-high speed connections called backbones. This is the Internet.


 Plain Old Telephone Service


POTS, or plain old telephone service, is still the most widely used method of accessing the
Internet. Its primary disadvantage is its low speed, but in many cases this is made up for by its
wide availability. Most national Internet service providers have a large number of local access
numbers, and almost everyone still has a phone with a land line. In theory, if you had an
acoustic modem and a pocket full of change, you could connect from almost any public
pay phone. Not that you would really want to do that.

POTS is slow. The fastest telephone modems are rated at a speed of 56,600 baud. That,
however, as they explain in the small print, is a lie. Power constraints limit the actual download
speed to about 53,000 baud and the effective rate is usually much lower. This doesn't
compare very well with DSL or cable modems.

That said, telephone service is widely available, and POTS based ISPs are relatively cheap
(and sometimes free). You wouldn't want to trade pirated movies over POTS, because it's
immoral, illegal and ties up your phone line all night and maybe into the afternoon, but you
could certainly send friendly, text based e-mails to Granny. And if you used telnet, you could
even do it with a dusty DOS based machine that you pulled out of the basement.


 DSL


DSL or digital subscriber line, is a method of sending large amounts of information over the
wires that already exist for the POTS. Its main advantage over POTS is that it is much faster
than analog modems, and it provides a permanent connection. In addition, it allows you to
make and receive regular telephone calls while you are connected to the Internet. Its main
disadvantage is that its availability is limited by your proximity to the telephone company's
switching equipment – if you live too far down the line; you're out of luck.


 Cable Modems


Cable modems do not use the traditional telephone lines to connect to the Internet. Instead
they make use of the optical fiber lines that are used by cable companies to transmit digital
cable signals. Like DSL, cable modems allow you to make and receive regular telephone calls
while you are connected to the Internet, and they provide a permanent connection, but
cable modems are generally faster than DSL.

Cable modems have two basic flaws. The first is that cable modem access is a shared
resource, so your connection speeds will be decreased when there are other users in close
geographic proximity. The second is that cable modem access is only available in areas
where cable companies have installed the necessary fiber optic wiring.


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