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■    A value of 126 or less indicates a Class A address. The first octet is the network number; the next three, the host address.

■    A value of exactly 127 is reserved as a loopback test address. If you send a message to, the Ping doesn’t actually generate any network traffic. It does, however, test that TCP/IP is installed correctly. Using this number as a special test address has the unfortunate effect of wasting more than 24 million possible IP addresses.

■    A value of 128 through 191 is a Class B address. The first two octets are the network number, and the last two are the host address.

■    A value of 192 through 223 is a Class C address. The first three octets are the network address, and the last octet is the host address.

■    A value greater than 223 indicates a reserved address.

Three other special address types are 10.x.x.x,, and 172.16.x.x-172.31.x.x. These addresses are specified in RFC 1918 as being available to anyone who wants to use IP addressing on a private network, but does not want to connect to the Internet. Private addresses are those addresses that are not routed by Internet routers. Public addresses are those IP addresses that will be passed by Internet routers. You can use this address without the risk of compromising someone else’s registered network address.

Understanding Subnets

The IP addressing scheme provides a flexible solution to the task of addressing thousands of networks, but it is not without problems. The original designers did not envision the Internet growing as large as it has; at that time, a 32-bit address seemed so large that they quickly divided it into different classes of networks to facilitate routing rather than reserving more bits to manage the growth in network addresses. (Who ever thought we would need a PC with more than 640KB of memory?) To solve this problem, and to create a large number of new network addresses, another way of dividing the 32-bit address was developed, called subnetting.

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