How Does the Internet Work?
© 2002 Rus
Shuler @ Pomeroy
IT Solutions, all rights reserved [ copy of
Where to Begin? Internet Addresses
Protocol Stacks and Packets
The Internet Routing Hierarchy
Domain Names and Address Resolution
Internet Protocols Revisited
Application Protocols: HTTP and the World Wide Web
Application Protocols: SMTP and Electronic Mail
Transmission Control Protocol
How does the Internet work? Good question! The Internet's growth has
become explosive and it seems impossible to escape the bombardment of www.com's
seen constantly on television, heard on radio, and seen in magazines.
Because the Internet has become such a large part of our lives, a good
understanding is needed to use this new tool most effectively.
whitepaper explains the underlying infrastructure and technologies that
make the Internet work. It does not go into great depth, but covers
enough of each area to give a basic understanding of the concepts
involved. For any unanswered questions, a list of resources is provided
at the end of the paper. Any comments, suggestions, questions, etc. are
encouraged and may be directed to the author at email@example.com.
Because the Internet is a global network of computers each computer
connected to the Internet must have a unique address. Internet
addresses are in the form nnn.nnn.nnn.nnn where nnn must be a
number from 0 - 255. This address is known as an IP address. (IP stands
for Internet Protocol; more on this later.)
The picture below
illustrates two computers connected to the Internet; your computer with
IP address 184.108.40.206 and another computer with IP address 220.127.116.11. The
Internet is represented as an abstract object in-between. (As this paper
progresses, the Internet portion of Diagram 1 will be explained and
redrawn several times as the details of the Internet are exposed.)
If you connect to the Internet through an Internet Service
Provider (ISP), you are usually assigned a temporary IP address for the
duration of your dial-in session. If you connect to the Internet from a
local area network (LAN) your computer might have a permanent IP address
or it might obtain a temporary one from a DHCP (Dynamic Host
Configuration Protocol) server. In any case, if you are connected to the
Internet, your computer has a unique IP address.
|Check It Out - The
|If you're using
Microsoft Windows or a flavor of Unix and have a connection to
the Internet, there is a handy program to see if a computer on
the Internet is alive. It's called ping, probably after
the sound made by older submarine sonar systems.1 If
you are using Windows, start a command prompt window. If you're
using a flavor of Unix, get to a command prompt. Type ping
www.yahoo.com. The ping program will send a 'ping'
(actually an ICMP (Internet Control Message Protocol) echo
request message) to the named computer. The pinged computer will
respond with a reply. The ping program will count the time
expired until the reply comes back (if it does). Also, if you
enter a domain name (i.e. www.yahoo.com) instead of an IP
address, ping will resolve the domain name and display the
computer's IP address. More on domain names and address
So your computer is connected to the Internet and has a unique address.
How does it 'talk' to other computers connected to the Internet? An
example should serve here: Let's say your IP address is 18.104.22.168 and you
want to send a message to the computer 22.214.171.124. The message you want to
send is "Hello computer 126.96.36.199!". Obviously, the message must be
transmitted over whatever kind of wire connects your computer to the
Internet. Let's say you've dialed into your ISP from home and the
message must be transmitted over the phone line. Therefore the message
must be translated from alphabetic text into electronic signals,
transmitted over the Internet, then translated back into alphabetic
text. How is this accomplished? Through the use of a protocol stack.
Every computer needs one to communicate on the Internet and it is
usually built into the computer's operating system (i.e. Windows, Unix,
etc.). The protocol stack used on the Internet is refered to as the
TCP/IP protocol stack because of the two major communication protocols
used. The TCP/IP stack looks like this:
|Application Protocols Layer
||Protocols specific to applications such as WWW, e-mail, FTP,
|Transmission Control Protocol Layer
||TCP directs packets to a specific application on a computer
using a port number.
|Internet Protocol Layer
||IP directs packets to a specific computer using an IP
||Converts binary packet data to network signals and back.
(E.g. ethernet network card, modem for phone lines, etc.)
If we were to follow the path that the message "Hello computer 188.8.131.52!"
took from our computer to the computer with IP address 184.108.40.206, it would
happen something like this:
- The message would start at the top of the protocol stack on your
computer and work it's way downward.
- If the message to be sent is long, each stack layer that the
message passes through may break the message up into smaller chunks
of data. This is because data sent over the Internet (and most
computer networks) are sent in manageable chunks. On the Internet,
these chunks of data are known as packets.
- The packets would go through the Application Layer and continue
to the TCP layer. Each packet is assigned a port number.
Ports will be explained later, but suffice to say that many programs
may be using the TCP/IP stack and sending messages. We need to know
which program on the destination computer needs to receive the
message because it will be listening on a specific port.
- After going through the TCP layer, the packets proceed to the IP
layer. This is where each packet receives it's destination address,
- Now that our message packets have a port number and an IP
address, they are ready to be sent over the Internet. The hardware
layer takes care of turning our packets containing the alphabetic
text of our message into electronic signals and transmitting them
over the phone line.
- On the other end of the phone line your ISP has a direct
connection to the Internet. The ISPs router examines the
destination address in each packet and determines where to send it.
Often, the packet's next stop is another router. More on routers and
Internet infrastructure later.
- Eventually, the packets reach computer 220.127.116.11. Here, the
packets start at the bottom of the destination computer's TCP/IP
stack and work upwards.
- As the packets go upwards through the stack, all routing data
that the sending computer's stack added (such as IP address and port
number) is stripped from the packets.
- When the data reaches the top of the stack, the packets have
been re-assembled into their original form, "Hello computer
So now you know how packets travel from one computer to another over
the Internet. But what's in-between? What actually makes up the
Internet? Let's look at another diagram:
Here we see Diagram 1 redrawn with more detail. The physical
connection through the phone network to the Internet Service Provider
might have been easy to guess, but beyond that might bear some
The ISP maintains a pool of modems for their dial-in
customers. This is managed by some form of computer (usually a dedicated
one) which controls data flow from the modem pool to a backbone or
dedicated line router. This setup may be refered to as a port server, as
it 'serves' access to the network. Billing and usage information is
usually collected here as well.
After your packets traverse the phone network and your ISP's local
equipment, they are routed onto the ISP's backbone or a backbone the ISP
buys bandwidth from. From here the packets will usually journey through
several routers and over several backbones, dedicated lines, and other
networks until they find their destination, the computer with address
18.104.22.168. But wouldn't it would be nice if we knew the exact route our
packets were taking over the Internet? As it turns out, there is a
|Check It Out - The
|If you're using
Microsoft Windows or a flavor of Unix and have a connection to
the Internet, here is another handy Internet program. This one
is called traceroute and it shows the path your packets
are taking to a given Internet destination. Like ping, you must
use traceroute from a command prompt. In Windows, use tracert
www.yahoo.com. From a Unix prompt, type traceroute
www.yahoo.com. Like ping, you may also enter IP addresses
instead of domain names. Traceroute will print out a list of all
the routers, computers, and any other Internet entities that
your packets must travel through to get to their destination.
If you use traceroute, you'll notice that your packets must travel
through many things to get to their destination. Most have long names
such as sjc2-core1-h2-0-0.atlas.digex.net and
fddi0-0.br4.SJC.globalcenter.net. These are Internet routers that decide
where to send your packets. Several routers are shown in Diagram 3, but
only a few. Diagram 3 is meant to show a simple network structure. The
Internet is much more complex.
The Internet backbone is made up of many large networks which
interconnect with each other. These large networks are known as Network
Service Providers or NSPs. Some of the large NSPs are UUNet,
CerfNet, IBM, BBN Planet, SprintNet, PSINet, as well as others. These
networks peer with each other to exchange packet traffic. Each
NSP is required to connect to three Network Access Points or NAPs.
At the NAPs, packet traffic may jump from one NSP's backbone to another
NSP's backbone. NSPs also interconnect at Metropolitan Area Exchanges or MAEs.
MAEs serve the same purpose as the NAPs but are privately owned. NAPs
were the original Internet interconnect points. Both NAPs and MAEs are
referred to as Internet Exchange Points or IXs. NSPs also sell
bandwidth to smaller networks, such as ISPs and smaller bandwidth
providers. Below is a picture showing this hierarchical infrastructure.
This is not a true representation of an actual piece of the
Internet. Diagram 4 is only meant to demonstrate how the NSPs could
interconnect with each other and smaller ISPs. None of the physical
network components are shown in Diagram 4 as they are in Diagram 3. This
is because a single NSP's backbone infrastructure is a complex drawing
by itself. Most NSPs publish maps of their network infrastructure on
their web sites and can be found easily. To draw an actual map of the
Internet would be nearly impossible due to it's size, complexity, and
ever changing structure.
So how do packets find their way across the Internet? Does every
computer connected to the Internet know where the other computers are?
Do packets simply get 'broadcast' to every computer on the Internet? The
answer to both the preceeding questions is 'no'. No computer knows where
any of the other computers are, and packets do not get sent to every
computer. The information used to get packets to their destinations are
contained in routing tables kept by each router connected to the
Routers are packet switches. A router is usually
connected between networks to route packets between them. Each router
knows about it's sub-networks and which IP addresses they use. The
router usually doesn't know what IP addresses are 'above' it. Examine
Diagram 5 below. The black boxes connecting the backbones are routers.
The larger NSP backbones at the top are connected at a NAP. Under them
are several sub-networks, and under them, more sub-networks. At the
bottom are two local area networks with computers attached.
When a packet arrives at a router, the router examines the IP
address put there by the IP protocol layer on the originating computer.
The router checks it's routing table. If the network containing the IP
address is found, the packet is sent to that network. If the network
containing the IP address is not found, then the router sends the packet
on a default route, usually up the backbone hierarchy to the next
router. Hopefully the next router will know where to send the packet. If
it does not, again the packet is routed upwards until it reaches a NSP
backbone. The routers connected to the NSP backbones hold the largest
routing tables and here the packet will be routed to the correct
backbone, where it will begin its journey 'downward' through smaller and
smaller networks until it finds it's destination.
But what if you don't know the IP address of the computer you want to
connect to? What if the you need to access a web server referred to as www.anothercomputer.com?
How does your web browser know where on the Internet this computer
lives? The answer to all these questions is the Domain Name Service or DNS.
The DNS is a distributed database which keeps track of computer's names
and their corresponding IP addresses on the Internet.
connected to the Internet host part of the DNS database and the software
that allows others to access it. These computers are known as DNS
servers. No DNS server contains the entire database; they only contain a
subset of it. If a DNS server does not contain the domain name requested
by another computer, the DNS server re-directs the requesting computer
to another DNS server.
The Domain Name Service is structured as a hierarchy similar to
the IP routing hierarchy. The computer requesting a name resolution will
be re-directed 'up' the hierarchy until a DNS server is found that can
resolve the domain name in the request. Figure 6 illustrates a portion
of the hierarchy. At the top of the tree are the domain roots. Some of
the older, more common domains are seen near the top. What is not shown
are the multitude of DNS servers around the world which form the rest of
When an Internet connection is setup (e.g. for a LAN or
Dial-Up Networking in Windows), one primary and one or more secondary
DNS servers are usually specified as part of the installation. This way,
any Internet applications that need domain name resolution will be able
to function correctly. For example, when you enter a web address into
your web browser, the browser first connects to your primary DNS server.
After obtaining the IP address for the domain name you entered, the
browser then connects to the target computer and requests the web page
|Check It Out -
Disable DNS in Windows
|If you're using Windows
95/NT and access the Internet, you may view your DNS server(s)
and even disable them.
If you use Dial-Up Networking:
Open your Dial-Up Networking window (which can be found in
Windows Explorer under your CD-ROM drive and above Network
Neighborhood). Right click on your Internet connection and click
Properties. Near the bottom of the connection properties window
press the TCP/IP Settings... button.
If you have a permanent connection to the Internet:
Right click on Network Neighborhood and click Properties. Click
TCP/IP Properties. Select the DNS Configuration tab at the top.
You should now be looking at your DNS servers' IP addresses.
Here you may disable DNS or set your DNS servers to 0.0.0.0.
(Write down your DNS servers' IP addresses first. You will
probably have to restart Windows as well.) Now enter an address
into your web browser. The browser won't be able to resolve the
domain name and you will probably get a nasty dialog box
explaining that a DNS server couldn't be found. However, if you
enter the corresponding IP address instead of the domain name,
the browser will be able to retrieve the desired web page. (Use
ping to get the IP address prior to disabling DNS.) Other
Microsoft operating systems are similar.
As hinted to earlier in the section about protocol stacks, one may
surmise that there are many protocols that are used on the Internet.
This is true; there are many communication protocols required for the
Internet to function. These include the TCP and IP protocols, routing
protocols, medium access control protocols, application level protocols,
etc. The following sections describe some of the more important and
commonly used protocols on the Internet. Higher level protocols are
discussed first, followed by lower level protocols.
Application Protocols: HTTP and the World Wide Web
One of the most commonly used services on the Internet is the World
Wide Web (WWW). The application protocol that makes the web work is Hypertext
Transfer Protocol or HTTP. Do not confuse this with the
Hypertext Markup Language (HTML). HTML is the language used to write web
pages. HTTP is the protocol that web browsers and web servers use to
communicate with each other over the Internet. It is an application
level protocol because it sits on top of the TCP layer in the protocol
stack and is used by specific applications to talk to one another. In
this case the applications are web browsers and web servers.
HTTP is a
connectionless text based protocol. Clients (web browsers) send requests
to web servers for web elements such as web pages and images. After the
request is serviced by a server, the connection between client and
server across the Internet is disconnected. A new connection must be
made for each request. Most protocols are connection oriented. This
means that the two computers communicating with each other keep the
connection open over the Internet. HTTP does not however. Before an HTTP
request can be made by a client, a new connection must be made to the
When you type a URL into a web browser, this is what happens:
- If the URL contains a domain name, the browser first connects to
a domain name server and retrieves the corresponding IP address for
the web server.
- The web browser connects to the web server and sends an HTTP
request (via the protocol stack) for the desired web page.
- The web server receives the request and checks for the desired
page. If the page exists, the web server sends it. If the server
cannot find the requested page, it will send an HTTP 404 error
message. (404 means 'Page Not Found' as anyone who has surfed the
web probably knows.)
- The web browser receives the page back and the connection is
- The browser then parses through the page and looks for other
page elements it needs to complete the web page. These usually
include images, applets, etc.
- For each element needed, the browser makes additional
connections and HTTP requests to the server for each element.
- When the browser has finished loading all images, applets, etc.
the page will be completely loaded in the browser window.
|Check It Out - Use
Your Telnet Client to Retrieve a Web Page Using HTTP
|Telnet is a remote
terminal service used on the Internet. It's use has declined
lately, but it is a very useful tool to study the Internet. In
Windows find the default telnet program. It may be located in
the Windows directory named telnet.exe. When opened, pull down
the Terminal menu and select Preferences. In the preferences
window, check Local Echo. (This is so you can see your HTTP
request when you type it.) Now pull down the Connection menu and
select Remote System. Enter www.google.com for the Host Name and
80 for the Port. (Web servers usually listen on port 80 by
default.) Press Connect. Now type
and press Enter twice. This is a simple HTTP request to a web
server for it's root page. You should see a web page flash by
and then a dialog box should pop up to tell you the connection
was lost. If you'd like to save the retrieved page, turn on
logging in the Telnet program. You may then browse through the
web page and see the HTML that was used to write it.
Most Internet protocols are specified by Internet documents known as a Request
For Comments or RFCs. RFCs may be found at several locations
on the Internet. See the Resources section below for appropriate URL's.
HTTP version 1.0 is specified by RFC 1945.
Protocols: SMTP and Electronic Mail
Another commonly used Internet service is electronic mail. E-mail
uses an application level protocol called Simple Mail Transfer
Protocol or SMTP. SMTP is also a text based protocol, but
unlike HTTP, SMTP is connection oriented. SMTP is also more complicated
than HTTP. There are many more commands and considerations in SMTP than
there are in HTTP.
When you open your mail client to read your e-mail,
this is what typically happens:
A simple 'conversation' between an SMTP client and SMTP server is shown
below. R: denotes messages sent by the server (receiver) and S: denotes
messages sent by the client (sender).
- The mail client (Netscape Mail, Lotus Notes, Microsoft Outlook,
etc.) opens a connection to it's default mail server. The mail
server's IP address or domain name is typically setup when the mail
client is installed.
- The mail server will always transmit the first message to
- The client will send an SMTP HELO command to which the server
will respond with a 250 OK message.
- Depending on whether the client is checking mail, sending mail,
etc. the appropriate SMTP commands will be sent to the server, which
will respond accordingly.
- This request/response transaction will continue until the client
sends an SMTP QUIT command. The server will then say goodbye and the
connection will be closed.
This SMTP example shows mail sent by Smith at host USC-ISIF, to
Jones, Green, and Brown at host BBN-UNIX. Here we assume that
host USC-ISIF contacts host BBN-UNIX directly. The mail is
accepted for Jones and Brown. Green does not have a mailbox at
R: 220 BBN-UNIX.ARPA Simple Mail Transfer Service Ready
S: HELO USC-ISIF.ARPA
R: 250 BBN-UNIX.ARPA
S: MAIL FROM:<Smith@USC-ISIF.ARPA>
R: 250 OK
S: RCPT TO:<Jones@BBN-UNIX.ARPA>
R: 250 OK
S: RCPT TO:<Green@BBN-UNIX.ARPA>
R: 550 No such user here
S: RCPT TO:<Brown@BBN-UNIX.ARPA>
R: 250 OK
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
R: 250 OK
R: 221 BBN-UNIX.ARPA Service closing transmission channel
This SMTP transaction is taken from RFC 821, which specifies
Transmission Control Protocol
Under the application layer in the protocol stack is the TCP layer.
When applications open a connection to another computer on the Internet,
the messages they send (using a specific application layer protocol) get
passed down the stack to the TCP layer. TCP is responsible for
routing application protocols to the correct application on the
destination computer. To accomplish this, port numbers are used.
Ports can be thought of as seperate channels on each computer. For
example, you can surf the web while reading e-mail. This is because
these two applications (the web browser and the mail client) used
different port numbers. When a packet arrives at a computer and makes
its way up the protocol stack, the TCP layer decides which application
receives the packet based on a port number.
TCP works like this:
This is how TCP routes the data moving through the protocol stack to the
- When the TCP layer receives the application layer protocol data
from above, it segments it into manageable 'chunks' and then adds a
TCP header with specific TCP information to each 'chunk'. The
information contained in the TCP header includes the port number of
the application the data needs to be sent to.
- When the TCP layer receives a packet from the IP layer below it,
the TCP layer strips the TCP header data from the packet, does some
data reconstruction if necessary, and then sends the data to the
correct application using the port number taken from the TCP header.
TCP is not a textual protocol. TCP is a
connection-oriented, reliable, byte stream service.
Connection-oriented means that two applications using TCP must first
establish a connection before exchanging data. TCP is reliable because
for each packet received, an acknowledgement is sent to the sender to
confirm the delivery. TCP also includes a checksum in it's header for
error-checking the received data. The TCP header looks like this:
Notice that there is no place for an IP address in the TCP
header. This is because TCP doesn't know anything about IP addresses.
TCP's job is to get application level data from application to
application reliably. The task of getting data from computer to computer
is the job of IP.
|Check It Out - Well
Known Internet Port Numbers
|Listed below are the
port numbers for some of the more commonly used Internet
|Quake III Arena
Unlike TCP, IP is an unreliable, connectionless protocol. IP
doesn't care whether a packet gets to it's destination or not. Nor does
IP know about connections and port numbers. IP's job is too send and
route packets to other computers. IP packets are independent
entities and may arrive out of order or not at all. It is TCP's job to
make sure packets arrive and are in the correct order. About the only
thing IP has in common with TCP is the way it receives data and adds
it's own IP header information to the TCP data. The IP header looks like
Above we see the IP addresses of the sending and receiving
computers in the IP header. Below is what a packet looks like after
passing through the application layer, TCP layer, and IP layer. The
application layer data is segmented in the TCP layer, the TCP header is
added, the packet continues to the IP layer, the IP header is added, and
then the packet is transmitted across the Internet.
Now you know how the Internet works. But how long will it stay this way?
The version of IP currently used on the Internet (version 4) only allows
232 addresses. Eventually there won't be any free IP
addresses left. Surprised? Don't worry. IP version 6 is being tested
right now on a research backbone by a consortium of research
institutions and corporations. And after that? Who knows. The Internet
has come a long way since it's inception as a Defense Department
research project. No one really knows what the Internet will become. One
thing is sure, however. The Internet will unite the world like no other
mechanism ever has. The Information Age is in full stride and I am glad
to be a part of it.
Rus Shuler, 1998
Below are some interesting links associated with some of the topics
discussed. (I hope they all still work. All open in new window.)
Updates made 2002
the home page of the Internet Engineering Task Force. This body is
greatly responsible for the development of Internet protocols and the
http://www.internic.org/ is the organization responsible for
administering domain names.
http://www.nexor.com/public/rfc/index/rfc.html is an excellent RFC
search engine useful for finding any RFC.
http://www.internetweather.com/ shows animated maps of Internet
http://routes.clubnet.net/iw/ is Internet Weather from ClubNET. This
page shows packet loss for various carriers.
http://navigators.com/isp.html is Russ Haynal's ISP Page. This is a
great site with links to most NSPs and their backbone infrastructure
The following books are excellent resources and helped greatly in the
writing of this paper. I believe Stevens' book is the best TCP/IP
reference ever and can be considered the bible of the Internet.
Sheldon's book covers a much wider scope and contains a vast amount of
Although not used for writing this paper, here are some other good books
on the topics of the Internet and networking:
- TCP/IP Illustrated, Volume 1, The Protocols.
W. Richard Stevens.
Addison-Wesley, Reading, Massachusetts. 1994.
- Encyclopedia of Networking.
Osbourne McGraw-Hill, New York. 1998.
- Firewalls and Internet Security; Repelling the Wiley Hacker.
William R. Cheswick, Steven M. Bellovin.
Addison-Wesley, Reading, Massachusetts. 1994.
- Data Communications, Computer Networks and Open Systems. Fourth
Addison-Wesley, Harlow, England. 1996.
- Telecommunications: Protocols and Design.
John D. Spragins with Joseph L. Hammond and Krzysztof Pawlikowski.
Addison-Wesley, Reading, Massachusetts. 1992.