Port Translation (NAPT), which allow multiple systems to share a single IP address by using different "port numbers." Port numbers are used by transport layer protocols to identify specific higher layer applications. Addressing in IP: The Domain Name System (DNS) IP addresses are 32-bits long, and while not all that complicated, most Internet users don't bother to memorize the dotted decimal addresses of their systems. Instead, they use natural language host names. Most hosts, then, must maintain a comparative "table" of both numeric IP addresses and natural language names. From a host perspective, how- ever, the names are worthless; they must use the numeric identifiers for routing purposes. Because the Internet continues to grow at a rapid clip, a system was needed to manage the growing list of new Internet domains. That system is the DNS. It is a distributed database that stores host names and IP address information for all of the recognized domains found on the Inter- net. For every domain there is an authoritative name server that contains all DNS-related information about that domain, and every domain has at least one secondary name server that also contains the information. A total of thirteen root servers, in turn, maintain a list of all of the author- itative name servers. How does the DNS actually work? When a system needs another sys- tem's IP address based upon its host name, the inquiring system issues a DNS request to a local name server. Depending on the contents of its local database, the local name server may be able to respond to the request. If not, it forwards the request to a root server. The root server, in turn, consults its own database and determines the most likely name server for the request and forwards it appropriately. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Transport Technologies Transport Technologies 