Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 41
Class A Address

Class A addresses take the 4–byte form <network number.host.host.host>, bytes 0, 1, 2, and 3. Subnetting has not been introduced here yet! Class A addresses use only the first of the 4 bytes for the network number. Class A is identified by the first bit in the first byte of the address. If this first bit is a 0, then it identifies a Class A address. The last 3 bytes are used for the host portion of the address.

Class A addressing allows for 126 networks (using only the first byte) with up to 16,777,214 million hosts per network number. The range for Class A is 1–126. With 24 bits in the host fields (last 3 bytes), there can be 16,277,214 hosts per network (again, disregarding subnets). This is actually (2ⁿ24) – 2. We subtract 2 because no host can be assigned all 0s (reserved to indicate a default route, which will be explained later) and no host can be assigned all 1s. For example, 10.255.255.255 is not allowed to be assigned to a host, although it is a valid address. Yes, this is a broadcast address.

If all 7 bits are set to 1 (starting from the right), this represents 127 in decimal, and 127.x.x.x is reserved as an internal loopback address and cannot be assigned to any host as a unique address. This is used to indicate whether your local TCP/IP stack (software) is up and running. The address is never seen on the network. You may want to look at your machine IP addresses (usually by typing netstat –r at the command line) and you will notice that every machine has 127.0.0.1 assigned to it. The software uses this as an internal loopback address. You should not see this address cross over the LAN (via a protocol analyzer such as a Sniffer.) In fact, 127.anything is proposed as the loopback. 127.1.1.1 delivers the same results as 127.0.0.1. Think about it. A whole address range assigned to one function: loopback. The problem is, if we tried to change it, it would probably cause mayhem on the millions of hosts that currently use IP.

Class A Address

Today, Class A addresses are being handed out through a different method involving Internet Service Providers that uses the Classless InterDomain Routing Protocol (CIDR), which is explained at the end of this section. When you get a Class A address, you will be told to subnet it appropriately (you will be told what the subnet address is). You will not get the whole Class A address. A good question here: How much of the address space does a Class A address define? (Hint: Do not think of it as a Class address but do use the first bit to answer the question). Give up?