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Class A addresses are designed for very large networks. In a Class A address, the first octet of the address is the network ID, and the remaining three octets are the host ID. Because only eight bits are allocated to the network ID and the first of these bits is used to indicate that the address is a Class A address, only 126 Class A networks can exist in the entire Internet. However, each Class A network can accommodate more than 16 million hosts.

Only about 40 Class A addresses are actually assigned to companies or organizations. The rest are either reserved for use by the Internet Assigned Numbers Authority (IANA) or are assigned to organizations that manage IP assignments for geographic regions such as Europe, Asia, and Latin America.

In case you’re interested, you can find a complete list of all the Class A address assignments at www.iana.org/assignments/ipv4-address-space/ipv4-address-space.xml.

You may have noticed in Table 3-3 that Class A addresses end with 126.x.y.z, and Class B addresses begin with 128.x.y.z. What happened to 127.x.y.z? This special range of addresses is reserved for loop-back testing, so these addresses aren't assigned to public networks.

The special address 127.0.0.1 is called the loop-back address. A device at any IP address that sends a message to 127.0.0.1 is sending a message to itself. This may sound useless, but it actually plays an important role in troubleshooting network problems.