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What is the Internet?
(Adapted from http://www.lehigh.edu/~inimr/webbasicsorig.PDF)
"The Internet” (capital "I" this time) is
the global information system consisting of computers on various
interconnected networks using the TCP/IP family of protocols.
There are many other large networks (such as those operated by
America Online and CompuServe, among others), which, whether they
connect to the Internet or not, are not part of it (because they
are based upon protocols other than TCP/IP). There are also private
TCP/IP-based networks that are not connected to the Internet.
1963: ASCII Debuts:
If it weren't for a particular development in 1963, we wouldn't
have e-mail and there would be no World Wide Web. Cursor movement,
laser printers and video games — all of these owe a big debt
of gratitude to this technological breakthrough. What is it? Something
most of us take for granted today: ASCII.
TCP/IP Protocol Family
The layers used in the TCP/IP family of protocols are shown in
the table below. In fact, the family takes its name from two protocols
at different levels: the transport-level protocol TCP, or "Transmission
Control Protocol" and the network-level protocol IP, or "Internet
Protocol." The protocols most users deal with directly, naturally,
are the application-level protocols. Occasionally, however, it
is useful to be aware of the other layers.
| Application |
- |
Telnet, FTP, SMTP, etc. |
| Transport |
- |
TCP, UDP |
| Network |
- |
IP |
| Link |
- |
Ethernet, Token-Ring, PPP |
Starting from the bottom and working upward...
Link-layer protocols deal with physically interfacing with the medium
(such as coaxial cable) used to interconnect the computers into
a network. This includes the operating system device driver software
and the network interface card. Notice that different portions of
the Internet may actually be built upon different physical network
types. Network-layer protocols deal with assigning addresses to
all of the computers on the Internet, and with the movement of packets
between source and destination. Routing of packets takes place at
this level. (There are some other protocols at this level, such
as ICMP, that are used for managing routers.) Transport-layer protocols
deal with data flow between computers. Dividing messages into packets,
reassembling messages from packets, acknowledging receipt of packets
or arranging for retransmission of missing or damaged packets, all
take place here. TCP, in particular, uses the IP packet system to
create reliable connections. (UDP provides faster, but not necessarily
completely reliable, connections for specialized purposes like real-time
audio or video, where occasional packet loss would not be disastrous.)
Application-layer protocols deal with the specifics of a particular
application, such as electronic mail. Each application corresponds
to a particular type of service supported by the Internet.
Client-Server Computing
An important aspect of most Internet applications is the fact that
they are all based on a client-server model. This is basically a
divide-and-conquer strategy for managing information and communication
resources. In this division of labor, one program, known as "the
server" is seen as holding, or controlling access to, some
resource (or alternatively as providing some service). Another program
(usually, but not always, on another computer) known as "the
client" is seen as making a request for, and becoming the recipient
of, this resource or service. Notice that the complete application
involves both the client and the server; neither alone is sufficient.
Typically, the user interacts directly with the client program on
his or her local computer, and uses it to retrieve information from
some remote computer that is running the server program. Often,
the server program is little more than a robot: it is continuously
running, and it sits and waits for incoming requests, which it then
attempts to fulfill. The "conversation" between client
and server is a series of requests and responses: the client asks
for something, and the server either satisfies the request (by providing
whatever-it-is that was asked-for) or returns an error message indicating
why it can't.
Sideline: Domain Name Services
One of the most fundamental of the underlying Internet services
is one most users take for granted; they usually aren't aware they
are using a service at all. This is DNS, the Domain Name Services
system. DNS is what makes it possible to specify a computer by name,
for example "jade.coe.edu", instead of by the ip number
(110.133.xx.xxx for our web server Jade). In the discussion above,
we mention that addresses are assigned to individual computers throughout
the Internet at the network layer. These addresses are known as
IP addresses, and all routing of packets from one computer to another
on the Internet uses them to specify source and destination. Whenever
you specify a particular computer by name, this name must first
be translated (the technical term is "resolved") into
an IP address. So, for example, if you try to connect to a remote
computer by name using Telnet, you are actually using two services:
Telnet and DNS. Telnet requests DNS resolution to obtain an IP address
it can use to find the remote computer it is supposed to connect
to. There is no centralized listing of all of the computers on the
Internet, either by name or in any other form. DNS is a distributed
database of names. DNS servers at many sites around the globe contain
information about the computers at that site, plus information about
other nearby DNS servers which can be queried about computers outside
the site (Coe College maintains two official DNS servers as part
of its campus network, a primary and a backup). DNS servers also
cache (remember) addresses for computers that have been requested
recently, in case they are needed again. Requests for distant computers
are passed along until an address is found, a server is found that
can definitely assert that no such machine exists, or a specified
timeout period expires (this last case means that sometimes the
system will fail to find a distant computer which does exist--in
such instances, because of caching, trying again may solve the problem).
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Sideline: Local Area Networks
One common source of confusion about networking concerns the way
Local Area Networks (LANs) fit into the picture. It's not as hard
as it seems. There are several areas of difference, and they are
much more significant than the areas of overlap. LANs, per se, are
not considered to be part of the Internet, even though the same
physical hardware (Ethernet cable and network interface cards, mostly)
may be used to access both.For one thing, LANs are typically used
to share different kinds of resources than are made available by
Internet applications: LANs are often used to provide access to
shared applications programs (which are run directly from the LAN
as if they were on the user's hard drive), shared disk space for
common storage, and shared peripheral devices (such as printers
and scanners). LANs (as the name implies) are typically used for
creating smaller networks, usually within a single organization
(or department). They can be also be used to provide internal electronic
mail and messaging services. More importantly, a LAN uses different
protocols for moving information around the network than do Internet
applications. LANs are also packet-based, and may use the same link-layer
protocols (such as Ethernet), along with the same underlying hardware,
but all higher-level functions are provided using different protocols.
In Coe's case, where our LANs are based on Novell NetWare, the LAN
protocol is called IPX. Both types of packets (IP and IPX) circulate
through the network simultaneously; so a given portion of the network
may be both a subnet of the Internet and a LAN at the same time
(in fact, a single user may be actively using both types of functions
at the same time--for example, loading and running an application
program, such as Telnet, from the LAN in order to access an Internet
service), but that doesn't mean that these two things are the same
thing.
What is the World-Wide Web?
The World-Wide Web is a collection of documents and services, distributed
across the Internet, and linked together by hypertext links into
an interconnected whole. As you can see from this definition, the
web is not the same thing as the Internet, but it is definitely
related. It is a subset of the Internet, taken from a particular
point-of-view. Much of what is available via the web consists of
web documents (sometimes misleadingly referred to as "home
pages"). These are special multipart documents (documents which,
although they are viewed as a single entity, actually consist of
several separate and distinct files), which can incorporate hypertext
features (links that lead you from document to document) and multimedia
(special non-textual features, such as graphics, animations, video,
sound, and interactive elements, including specially-embedded programs).
The base document of such a multipart document is a file which is
mostly just text, plus some specialized commands (called "markup")
which describe the structure of the overall compound document and
determine where and how the other components (such as image files)
are to be embedded within it. It is this markup which also creates
the links between documents. The rules governing this markup forms
a simple language called HTML (which just stands for "Hyper
Text Markup Language"). However, the web incorporates more
than just a set of documents of a special type. It also provides
a way of accessing various Internet services, many of which were
not specifically designed for use with the web. It is therefore
also a new form of user interface to many different types of information
available on the Internet.
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