A: The Domain Name System or DNS exists solely to enable the users of TCP/IP systems to identify other systems using friendly host names like www.zacker.com instead of IP addresses. TCP/IP communications are based on Internet Protocol (or IP) datagrams, which function as the "envelope" that contains the message being delivered. IP datagrams identify the systems that are the source and the destination of each message using only IP addresses. When you specify a host name in a client program, it is the responsibility of the DNS server to convert that host name into an IP address before any communication with that host can begin. This process is called name resolution.

Whenever your system must resolve a host name, it transmits a request to the DNS server specified in your TCP/IP client configuration. There are thousands of DNS servers on the Internet and many millions of host names, so it is impossible for each server to maintain a complete and up-to-date record of all the names on the net. Instead, the DNS consists of a distributed database, meaning that host names and their equivalent IP addresses are scattered on DNS servers located all over the world. There is no single server anywhere that maintains a complete record.

This distribution of data is organized by domain names, which conform a hierarchy of domain servers. When your DNS server attempts to resolve the name www.zacker.com, it works from right to left, starting at the highest level domain name: com. Every DNS server is configured with the addresses of other servers at the top of the DNS hierarchy. These servers handle hundreds of requests per second, but all they do is help the requesting server to locate the next highest domain in the host name. In other words, your DNS server queries the com server for information about the zacker domain, and receives in reply the address of another DNS server that has been registered as the authoritative server for the zacker.com domain. This server is the one where I (or my service provider) defines the hosts in my domain and specifies their IP addresses. Your DNS server, having received the address of my DNS server from the com domain, then transmits a DNS request to my server, asking for the IP address of the host system www in the zacker.com domain. My server replies to your server with the requested information, and your server passes the IP address to your own system. It is only after this name resolution process is completed that your system can transmit its first datagram directly to the www.zacker.com machine.

For more information, see page 396 of TCP/IP Administration.