work. IP Address Classes IP defines distinct address classes, which are used to discriminate between different sized networks. Classes A, B, and C are used for host addressing; the only difference between them is the length of the NET_ID subfield. A Class A address, for example, has an eight-bit NET_ID field and a 24-bit HOST_ID field. They are used for identifica- tion of very large networks and can identify as many as 16,777,214 hosts in each network. To date, only about 90 Class A addresses have been assigned. Class B addresses have 16-bit NET_ID and 16-bit HOST ID fields. They are used to address medium-sized-networks and can identify as many as 65,534 hosts within each network. Class C addresses, which are far and away the most common, have a 24-bit NET_ID field and an eight-bit HOST_ID field. These addresses are used for smaller networks and can identify no more than 254 devices within any given network. There are 2,097,152 possible Class C NET_IDs which are commonly assigned to corporations that have fewer than 250 employees (or devices!). There are two additional address types. Class D addresses are used for IP multicasting, such as transmitting a television signal to multiple recipients, whereas Class E addresses are reserved for experimental use. 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 Some addresses are reserved for specific purposes. A HOST_ID of 0 is reserved to identify an entire subnetwork. For example, the address 168.152.20.0 refers to a Class C address with a NET_ID of 168.152.20. A HOST_ID that consists of all ones (usually written "255" when referring to an all-ones byte, but also denoted as "-1") is reserved as a broadcast address and is used to transmit a message to all hosts on a particular network. Subnet Masking One of the most valuable but least understood tools 