Let's say that you are sitting at your computer, surfing the Web, and you get a call from a friend who says, "I just read a great article! Type in this URL and check it out! It's at http://www.Sitewarehouse.com/how.html." So you type that URL (Unique Resource Locator) into your browser and press return. And magically, no matter where in the world that URL lives, the page pops up on your screen!

At the most basic level possible, the following diagram shows the steps that brought that page to your screen:

Your browser formed a connection to a Web server, requested a page and received it. If you want to get into a bit more detail, here are the basic steps that occurred behind the scenes:

The browser broke the URL into three parts:
The protocol ("http")
The server name ("www.Sitewarehouse.com")
The file name ("/how.html")

The browser communicated with a name server to translate the server name "www.Sitewarehouse.com" into an IP Address, which it uses to connect to the server machine.

The browser then formed a connection to the server at that IP address on port 80. The number 80 represents the path that is open on the server to accept requests from other computers that are limited to the http protocol.

Following the HTTP protocol, the browser sent a GET request to the server, asking for the file "http://www.Sitewarehouse.com/how.html." (Note that cookies may be sent from browser to server with the GET request -- see How Internet Cookies Work for details.)

The server then sent the HTML text for the Web page to the browser. (Cookies may also be sent from server to browser in the header for the page.)

The browser read the HTML tags and formatted the page onto your screen.

If you've never explored this process before, that's a lot of new vocabulary. To understand this whole process in detail, you need to learn about IP addresses, ports, protocols... The following sections will lead you through a complete explanation.

The Internet
So what is "the Internet"? The Internet is a gigantic collection of millions of computers, all linked together on a computer network. The network allows all of the computers to communicate with one another. A home computer may be linked to the Internet using a phone-line modem, DSL or cable modem that talks to an Internet service provider (ISP). A computer in a business or university will usually have a network interface card (NIC) that directly connects it to a local area network (LAN) inside the business. The business can then connect its LAN to an ISP using a high-speed phone line like a T1 line. A T1 line can handle approximately 1.5 million bits per second, while a normal phone line using a modem can typically handle 30,000 to 50,000 bits per second.

ISPs then connect to larger ISPs, and the largest ISPs maintain fiber-optic "backbones" for an entire nation or region. Backbones around the world are connected through fiber-optic lines, undersea cables or satellite links (see this page for a nice backbone and connection diagram). In this way, every computer on the Internet is connected to every other computer on the Internet.

In general, all of the machines on the Internet can be categorized as two types: servers and clients. Those machines that provide services (like Web servers or FTP servers) to other machines are servers. And the machines that are used to connect to those services are clients. When you connect to Yahoo! at www.yahoo.com to read a page, Yahoo! is providing a machine (probably a cluster of very large machines), for use on the Internet, to service your request. Yahoo! is providing a server. Your machine, on the other hand, is probably providing no services to anyone else on the Internet. Therefore, it is a user machine, also known as a client. It is possible and common for a machine to be both a server and a client, but for our purposes here you can think of most machines as one or the other.

A server machine may provide one or more services on the Internet. For example, a server machine might have software running on it that allows it to act as a Web server, an e-mail server and an FTP server. 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 overall server machine. For example, if you are running a Web browser on your machine, it will most likely want to talk to the Web server on the server machine. Your Telnet application will want to talk to the Telnet server, your e-mail application will talk to the e-mail server, and so on...

IP Addresses
To keep all of these machines straight, each machine on the Internet is assigned a unique address called an IP address. IP stands for Internet protocol, and these addresses are 32-bit numbers, normally expressed as four "octets" in a "dotted decimal number." A typical IP address looks like this:

216.27.61.137

The four numbers in an IP address are called octets because they can have values between 0 and 255, which is 28 possibilities per octet.

Every machine on the Internet has a unique IP address. A server has a static IP address that does not change very often. A home machine that is dialing up through a modem often has an IP address that is assigned by the ISP when the machine dials in. That IP address is unique for that session -- it may be different the next time the machine dials in. This way, an ISP only needs one IP address for each modem it supports, rather than for each customer.

As far as the Internet's machines are concerned, an IP address is all you need to talk to a server. For example, in your browser, you can type the URL http://209.116.69.66 and arrive at the same machine that contains the Web server fora site. On some servers, the IP address alone is not sufficient, but on most large servers it is -- keep reading for details.

Name Servers
Because most people have trouble remembering the strings of numbers that make up IP addresses, and because IP addresses sometimes need to change, all servers on the Internet also have human-readable names, called domain names. For example, www.Sitewarehouseing.com is a permanent, human-readable name. It is easier for most of us to remember www.Sitewarehouse.com than it is to remember 209.116.69.66.
The name www.Sitewarehouse.com actually has three parts:

The host name ("www")
The domain name ("Sitewarehouse")
The top-level domain name ("com")

Domain names are managed by a company called ICANN/InterNic. ICANN creates the top-level domain names and guarantees that all names within a top-level domain are unique. NSI Inc. maintains contact information for each site and runs the "whois" database on behalf of ICANN/InterNic. The host name is created by the company hosting the domain. "www" is a very common host name, but many places now either omit it or replace it with a different host name that indicates a specific area of the site. For example, in encarta.msn.com, the domain name for Microsoft's Encarta encyclopedia, "encarta" is designated as the host name instead of "www."

The whois Command
On a UNIX machine, you can use the whois command to look up information about a domain name. You can do the same thing using the whois form at www.internic.com. If you type in a domain name, like "Sitewarehouse.com," it will return to you the registration information for that domain, including its IP address.

A set of servers called domain name servers (DNS) maps the human-readable names to the IP addresses. These servers are simple databases that map names to IP addresses, and they are distributed all over the Internet. Most individual companies, ISPs and universities maintain small name servers to map host names to IP addresses. There are also central name servers that use data supplied by ICANN to map domain names to IP addresses.

If you type the URL "http://www.Sitewarehouse.com/how.html" into your browser, your browser extracts the name "www.Sitewarehouse.com," passes it to a domain name server, and the domain name server returns the correct IP address for www.Sitewarehouse.com. A number of name servers may be involved to get the right IP address. For example, in the case of www.Sitewarehouse.com, the name server for the "com" top-level domain will know the IP address for the name server that knows host names, and a separate query to that name server, operated by the ISP, may deliver the actual IP address for the Sitewarehouse server machine.

So here it is: The Internet is made up of millions of machines, each with a unique IP address. Many of these machines are server machines, meaning that they provide services to other machines on the Internet. You have heard of many of these servers: e-mail servers, Web servers, FTP servers, Gopher servers and Telnet servers, to name a few. All of these are provided by server machines.