in IP protocol man- agement is called the subnet mask. Subnet masks are used to identify the portion of the address that specifies the network or the subnetwork for routing purposes. They also may be used to divide a large address into subnetworks or to combine multiple, smaller addresses to create a single large "domain." In the case of an organization subdividing its network, the address space is apportioned to identify multiple logical networks. This is accomplished by further dividing the HOST_ID subfield into a Subnetwork Identifier (SUBNET_ID) and a HOST_ID. Adding to the Alphabet Soup: CIDR, DHCP, NAT, and PAT As soon as the Internet became popular in the early 90s, concerns began to arise about the eventual exhaustion of available IP addresses. For example, consider what happens when a small corporation of 11 employ- ees purchases a Class C address. They now control more than 250 addresses, of which they may only be using 25. Clearly, this is a waste of a scarce resource. One technique that has been accepted for "address space conserva- tion" is called Classless Interdomain Routing (CIDR). CIDR effectively limits the number of addresses assigned to a given organization, making the process of address assignment far more granular- and therefore efficient. Furthermore, CIDR has had a secondary, yet equally important, impact: It has dramatically reduced the size of the Internet routing tables because of the preallocation techniques used for address space management. 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 Chapter 7 Other important protocols include Network Address Translation (NAT), which translates a private IP address that is being used to access the Web into a public IP address from an available pool of addresses, thus further conserving address space; Port Address Trans- lation (PAT) and Network Address 