Clients and Servers
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...
How Internet Cookies Work
Most Internet cookies are incredibly simple, but they are one of those things that have taken on a life of their own. Cookies started receiving tremendous media attention back in February 2000 because of Internet privacy concerns, and the debate still rages. On the other hand, cookies provide capabilities that make the Web much easier to navigate. The designers of almost every major site use them because they provide a better user experience and make it much easier to gather accurate information about the site's visitors.
To keep all of these computers 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: 126.96.36.199 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.
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.yourdomainname.com is a permanent, human-readable name. It is easier for most of us to remember www.yourdomainname.com than it is to remember 188.8.131.52. The name www.yourdomainname.com actually has three parts: The host name (\"www\") The domain name (\"yourdomainname\") The top-level domain name (\"com\") Domain names are managed by a company called ICANN. ICANN creates the top-level domain names and guarantees that all names within a top-level domain are unique. ICANN contracts a company to maintain contact information for each site and runs the \"whois\" database. 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.
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.