Java Network Programming, Second Edition

Chapter 6. Looking Up Internet Addresses

DNS, IP Addresses, and All That

Devices connected to the Internet are called nodes. Nodes that are computers are called hosts. Each node or host is identified by at least one unique 32-bit number called an Internet address, an IP address, or a host address, depending on who you talk to. This takes up exactly four bytes of memory. An IP address is normally written as four unsigned bytes, each ranging from to 255, with the most significant byte first. Bytes are separated by periods for the convenience of human eyes. For example, the address for hermes.oit.unc.edu is 152.2.21.1. This is called the dotted quad format.

IP addresses are great for computers, but they are a problem for humans, who have a hard time remembering long numbers. In the 1950s, it was discovered that most people could remember about seven digits per number; some can remember as many as nine, while others remember as few as five. This is why phone numbers are broken into three- and four-digit pieces with three-digit area codes.^[13] Obviously an IP address, which can have as many as 12 decimal digits, is beyond the capacity of most humans to remember. I can remember about two IP addresses, and then only if I use both daily and the second is a simple permutation of the first.

To avoid the need to carry around Rolodexes full of IP addresses, the designers of the Internet invented the Domain Name System (DNS). DNS associates hostnames that humans can remember (like hermes.oit.unc.edu ...