|  | 8.6 | TCP/IP Fundamentals
| 8.6.6 | Class A, B, and C addresses and their default subnet mask numbers
IP addresses are divided into classes to accommodate different sizes of networks. Depending on the type of IP address, the first octets (through the first, second, or third octets) are issued by ARIN or other national agencies. The IP address has 3 parts: The network field (assigned externally), the subnetwork field (created by the network administrator locally), and the host field (assigned locally). In order to create more hierarchical networks with subnets, the extended network prefix, also known as the subnet mask, which allows the decoding of the IP address into its 3 parts (network, subnetwork, host) is required. The classes of IP addresses are as follows: Class A addresses begin with binary 0xxx xxxx in the first octet; that is, decimal 1 to 126 (0 and 127 are reserved for special test purposes). Class B Addresses begin with binary 10xx xxxx in the first octet; that is, decimal 128 to 191. Class C Addresses begin with binary 110x xxxx in the first octet; that is decimal 192 to 223. Class A IP addresses are reserved for the larger networks. This range allows Class A IP addresses to have a possibility of 126 networks, while each network has a capacity of more than 16 million unique hosts using default subnet mask. The default subnet mask for this class network is 255.0.0.0. Class B IP addresses are reserved for medium sized networks. The range dedicated for Class B networks includes IP addresses from 128.0.0.0 to 191.255.0.0. The possibility of networks available for this subnet is a little more than 16 thousand networks, with each having the capacity of a few more than 65 thousand unique hosts using the default subnet mask. The default subnet mask for a Class B address is 255.255.0.0. Class C IP addresses are dedicated for small Local Networks. Class C networks range from 192.0.1.0 to 223.255.255.0 with a capacity of more than 2 million networks, each with a capacity of 254 unique hosts using the default subnet mask. The default subnet mask for a Class C address is 255.255.255.0. Other classes, which are not as popular, are Classes D and E. Class D ranges from 224.0.0.0 to 239.255.255.255 and is used mainly for multicasting to various amounts of hosts. It has the potential of having more than 268 million unique multicast groups. Class E, which are experimental addresses block for future use, has a range of 240.0.0.0 to 247.255.255.255. Various solutions to the problem of IP address depletion are being implemented, including NAT (Network Address Translation) and Private Address; VLSM (variable length subnet masking), CIDR (classless interdomain routing); and IP v 6 (version 6 of IP with longer IP addresses, and hence far more to assign). -