The Internet

The Internet is simply the world's biggest computer network. It connects millions of hosts (computers, servers, devices, and so on) to each other so that they can communicate and interact.

The Internet is not a physical entity, nor is it any particular host or set of hosts. You could never point to a machine and identify it as the Internet, nor could you ever turn the Internet on or off. The Internet is a living entity, and one that is evolving and changing all the time.

IP

The Internet is held together by IP, the Internet Protocol, and every host connected to the Internet must be running a copy of IP.

IP requires every host to have a unique address by which to identify it. The unique identifiers are IP addresses that (in the current version of IP) are made up of four sets of numbers separated by periodsfor example, 65.36.166.120. Some hosts have fixed (or static) IP addresses; others have dynamically assigned addresses. Regardless of how an IP address is obtained, no two hosts connected to the Internet may use the same IP address at any given time. (The exception to this rule is the addressing used in a private network, which needs to be unique only within that network.)

NOTE

The IP address 127.0.0.1 always points to the local machine.

DNS

IP addresses are the only way to uniquely specify a host. When you want to communicate with, for example, a Web server host, you must specify the IP address of the Web server you're trying to contact. Similarly, when you connect to an FTP server, or specify the SMTP and POP servers in your mail client, you must specify the name of the host to which you want to connect.

As you know from browsing the Web, you rarely specify IP addresses directly. You do, however, specify a host name, such as www.forta.com. If hosts are identified by IP address, how does your browser know which Web server to contact when you specify a host name?

The answer is the Domain Name Service (DNS), a mechanism that maps host names to IP addresses. When you specify the destination address of www.forta.com, your browser sends an address resolution request to a DNS server, asking for the IP address of that host. The DNS server returns an actual IP addressin this case, 65.36.166.120. Your browser can then use this address to communicate with the host directly.

DNS is rarely required. Users can usually specify the name of a destination host by its IP address to connect to the host. There are, however, some very good reasons not to use the IP address:

• IP addresses are hard to remember and easy to mistype. Users are more likely to find www.forta.com than they are 65.36.166.120.

• IP addresses are subject to change. For example, if you switch service providers, you might be forced to use a new set of IP addresses for your hosts. If users identified your site only by its IP address, they could never reach your host if the IP address changed. Your DNS name stays the same, even if your IP address switches. You need to change only the mapping so that the host name maps to the new, correct IP address.

• Multiple hosts, each with unique IP addresses, can all have the same DNS name. This allows load balancing between servers, as well as the establishment of redundant servers.

• A single host, with a single IP address, can have multiple DNS names. This enables you to create aliases if needed. For example, ftp.forta.com and www.forta.com might point to the same IP address, and thus the same server.

NOTE

The host name localhost always points to the IP address 127.0.0.1, the local machine.