One of the greatest things about the Internet is that
nobody really owns it. It is a global collection of networks,
both big and small. These networks connect together in many
different ways to form the single entity that we know as the
Internet. In fact, the very name comes from this idea
of interconnected networks.
Since its beginning in 1969, the Internet has grown from
four host computer systems to tens of millions. However, just
because nobody owns the Internet, it doesn't mean it is not
monitored and maintained in different ways. The
Internet Society, a non-profit group established in 1992,
oversees the formation of the policies and protocols that
define how we use and interact with the Internet.
In this article, you will learn about the basic underlying
structure of the Internet. You will learn about domain name
servers, network access points and backbones. But first you
will learn about how your computer connects to others.
A Hierarchy of Networks Every computer that
is connected to the Internet is part of a network, even the
one in your home. For example, you may use a modem
and dial a local number to connect to an Internet Service
Provider (ISP). At work, you may be part of a local
area network (LAN),
but you most likely still connect to the Internet using an ISP
that your company has contracted with. When you connect to
your ISP, you become part of their network. The ISP may then
connect to a larger network and become part of their network.
The Internet is simply a network of networks.
Most large communications companies have their own
dedicated backbones connecting various regions. In each
region, the company has a Point of Presence (POP). The
POP is a place for local users to access the company's
network, often through a local phone number or dedicated line.
The amazing thing here is that there is no overall controlling
network. Instead, there are several high-level networks
connecting to each other through Network Access Points
When you connect to the Internet, your
computer becomes part of a
A Network Example Here's an example. Imagine
that Company A is a large ISP. In each major city, Company A
has a POP. The POP in each city is a rack full of modems that
the ISP's customers dial into. Company A leases fiber
optic lines from the phone company to connect the POPs
together (see, for example, this UUNET Data Center Connectivity Map).
Imagine that Company B is a corporate ISP. Company B builds
large buildings in major cities and corporations locate their
Internet server machines in these buildings. Company B is such
a large company that it runs its own fiber optic lines between
its buildings so that they are all interconnected.
In this arrangement, all of Company A's customers can talk
to each other, and all of Company B's customers can talk to
each other, but there is no way for Company A's customers and
Company B's customers to intercommunicate. Therefore, Company
A and Company B both agree to connect to NAPs in various
cities, and traffic between the two companies flows between
the networks at the NAPs.
In the real Internet, dozens of large Internet providers
interconnect at NAPs in various cities, and trillions of bytes
of data flow between the individual networks at these points.
The Internet is a collection of huge corporate networks that
agree to all intercommunicate with each other at the NAPs. In
this way, every computer on the Internet connects to every
Bridging The Divide All of these networks
rely on NAPs, backbones and routers to talk to each
other. What is incredible about this process is that a message
can leave one computer and travel halfway across the world
through several different networks and arrive at another
computer in a fraction of a second!
determine where to send information from one computer to
another. Routers are specialized computers that send your
messages and those of every other Internet user speeding to
their destinations along thousands of pathways. A router has
two separate, but related, jobs:
It ensures that information doesn't go where it's not
needed. This is crucial for keeping large volumes of data
from clogging the connections of "innocent bystanders."
It makes sure that information does make it to the
In performing these two jobs, a router is extremely useful
in dealing with two separate computer networks. It joins the
two networks, passing information from one to the other. It
also protects the networks from one another, preventing the
traffic on one from unnecessarily spilling over to the other.
Regardless of how many networks are attached, the basic
operation and function of the router remains the same. Since
the Internet is one huge network made up of tens of thousands
of smaller networks, its use of routers is an absolute
necessity. For more information, read How
Backbones The National Science
Foundation (NSF) created the first high-speed backbone in
1987. Called NSFNET, it was a T1 line that connected
170 smaller networks together and operated at 1.544 Mbps
per second). IBM, MCI and Merit worked with NSF to create the
backbone and developed a T3 (45 Mbps) backbone the following
Backbones are typically fiber optic trunk lines. The trunk
line has multiple fiber optic cables combined together to
increase the capacity. Fiber optic cables are designated OC
for optical carrier, such as OC-3, OC-12 or OC-48. An OC-3
line is capable of transmitting 155 Mbps while an OC-48 can
transmit 2,488 Mbps (2.488 Gbps). Compare that to a typical
56K modem transmitting 56,000 bps and you see just how fast a
modern backbone is.
Today there are many companies that operate their own
high-capacity backbones, and all of them interconnect at
various NAPs around the world. In this way, everyone on the
Internet, no matter where they are and what company they use,
is able to talk to everyone else on the planet. The entire
Internet is a gigantic, sprawling agreement between companies
to intercommunicate freely.
Internet Protocol: IP Addresses Every
machine on the Internet has a unique identifying number,
called an IP Address. The IP stands for Internet
Protocol, which is the language that computers use to
communicate over the Internet. A protocol is the pre-defined
way that someone who wants to use a service talks with that
service. The "someone" could be a person, but more often it is
a computer program like a Web browser.
A typical IP address looks like this:
To make it easier for us humans to remember, IP addresses
are normally expressed in decimal format as a dotted
decimal number like the one above. But computers
communicate in binary
form. Look at the same IP address in binary:
The four numbers in an IP address are called octets,
because they each have eight positions when viewed in binary
form. If you add all the positions together, you get 32, which
is why IP addresses are considered 32-bit numbers. Since each
of the eight positions can have two different states (1 or
zero), the total number of possible combinations per octet is
28 or 256. So each octet can
contain any value between zero and 255. Combine the four
octets and you get 232 or a
possible 4,294,967,296 unique values!
Out of the almost 4.3 billion possible combinations,
certain values are restricted from use as typical IP
addresses. For example, the IP address 0.0.0.0 is reserved for
the default network and the address 255.255.255.255 is used
The octets serve a purpose other than simply separating the
numbers. They are used to create classes of IP
addresses that can be assigned to a particular business,
government or other entity based on size and need. The octets
are split into two sections: Net and Host. The
Net section always contains the first octet. It is used to
identify the network that a computer belongs to. Host
(sometimes referred to as Node) identifies the actual
computer on the network. The Host section always contains the
last octet. There are five IP classes plus certain special
addresses. You can learn more about IP classes at What
is an IP address?.
Internet Protocol: Domain Name System When
the Internet was in its infancy, it consisted of a small
number of computers hooked together with modems and telephone
lines. You could only make connections by providing the IP
address of the computer you wanted to establish a link with.
For example, a typical IP address might be 188.8.131.52. This
was fine when there were only a few hosts out there, but it
became unwieldy as more and more systems came online.
The first solution to the problem was a simple text file
maintained by the Network Information Center that mapped names
to IP addresses. Soon this text file became so large it was
too cumbersome to manage. In 1983, the University of Wisconsin
created the Domain Name System (DNS), which maps text
names to IP addresses automatically. This way you only need to
for example, instead of HowStuffWorks.com's IP address.
Uniform Resource Locators When you use the
Web or send an e-mail message, you use a domain name to do it.
For example, the Uniform Resource Locator (URL) "http://computer.howstuffworks.com/"
contains the domain name howstuffworks.com. So does this
e-mail address: firstname.lastname@example.org. Every time you use
a domain name, you use the Internet's DNS servers to translate
the human-readable domain name into the machine-readable IP
address. Check out How Domain
Name Servers Work for more in-depth information on DNS.
Top-level domain names, also called first-level domain
names, include .COM, .ORG, .NET, .EDU and .GOV. Within every
top-level domain there is a huge list of second-level domains.
For example, in the .COM first-level domain there is:
Every name in the .COM top-level domain must be unique. The
left-most word, like www, is the host name. It specifies the
name of a specific machine (with a specific IP address) in a
domain. A given domain can, potentially, contain millions of
host names as long as they are all unique within that domain.
DNS servers accept requests from programs and other name
servers to convert domain names into IP addresses. When a
request comes in, the DNS server can do one of four things
It can answer the request with an IP address because it
already knows the IP address for the requested domain.
It can contact another DNS server and try to find the IP
address for the name requested. It may have to do this
It can say, "I don't know the IP address for the domain
you requested, but here's the IP address for a DNS server
that knows more than I do."
It can return an error message because the requested
domain name is invalid or does not exist.
A DNS Example Let's say that you type the
into your browser. The browser contacts a DNS server to get
the IP address. A DNS server would start its search for an IP
address by contacting one of the root DNS servers. The
root servers know the IP addresses for all of the DNS servers
that handle the top-level domains (.COM, .NET, .ORG, etc.).
Your DNS server would ask the root for www.howstuffworks.com,
and the root would say, "I don't know the IP address for
www.howstuffworks.com, but here's the IP address for the .COM
Your name server then sends a query to the .COM DNS server
asking it if it knows the IP address for
www.howstuffworks.com. The DNS server for the COM domain knows
the IP addresses for the name servers handling the http://computer.howstuffworks.com/
domain, so it returns those.
One of the keys to making this work is redundancy. There
are multiple DNS servers at every level, so that if one fails,
there are others to handle the requests. The other key is
caching. Once a DNS server resolves a request, it caches the
IP address it receives. Once it has made a request to a root
DNS server for any .COM domain, it knows the IP address for a
DNS server handling the .COM domain, so it doesn't have to bug
the root DNS servers again for that information. DNS servers
can do this for every request, and this caching
helps to keep things from bogging down.
Even though it is totally invisible, DNS servers handle
billions of requests every day and they are essential to the
Internet's smooth functioning. The fact that this distributed
database works so well and so invisibly day in and day out is
a testimony to the design. Be sure to read How Domain
Name Servers Work for more information on DNS.
Clients and Servers Internet servers
make the Internet possible. All of the machines on the
Internet are either servers or clients. The machines
that provide services to other machines are servers. And the
machines that are used to connect to those services are
clients. There are Web servers, e-mail servers, FTP servers
and so on serving the needs of Internet users all over the
When you connect to http://computer.howstuffworks.com/
to read a page, you are a user sitting at a client's machine.
You are accessing the HowStuffWorks Web server. The server
machine finds the page you requested and sends it to you.
Clients that come to a server machine do so with a specific
intent, so clients direct their requests to a specific
software server running on the server machine. For example, if
you are running a Web browser on your machine, it will want to
talk to the Web server on the server machine, not the e-mail
A server has a static IP address that does not change very
often. A home machine that is dialing up through a modem, on
the other hand, typically has an IP address assigned by the
ISP every time you dial in. That IP address is unique for your
session -- it may be different the next time you dial in. This
way, an ISP only needs one IP address for each modem it
supports, rather than one for each customer.
Ports Any server machine makes its services
available using numbered ports -- one for each service that is
available on the server. For example, if a server machine is
running a Web server and a file transfer protocol (FTP)
server, the Web server would typically be available on port
80, and the FTP server would be available on port 21. Clients
connect to a service at a specific IP address and on a
specific port number.
Once a client has connected to a service on a particular
port, it accesses the service using a specific protocol.
Protocols are often text and simply describe how the client
and server will have their conversation. Every Web server on
the Internet conforms to the hypertext transfer protocol
(HTTP). You can learn more about Internet servers, ports
and protocols by reading How
Web Servers Work.
Networks, routers, NAPs, ISPs, DNS and powerful servers all
make the Internet possible. It is truly amazing when you
realize that all this information is sent around the world in
a matter of milliseconds! The components are extremely
important in modern life -- without them, there would be no
Internet. And without the Internet, life would be very
different indeed for many of us.
For more information on the structure of the Internet and
related topics, check out the links on the next page.