When IP was first developed, no classes of addresses existed, because it was assumed that 254 networks would be more than enough for an internetwork of academic, military, and research computers.As the number of networks grew, the IP addresses were broken into categories called classes to accommodate different sizes of networks and to aid in identifying them.
Assigning IP addresses to classes is known as classful addressing. The allocation of addresses is managed by a central authority, the American Registry for Internet Numbers (ARIN), which you can go to at http://www.arin.net for more information about network numbers.
Assigning IP addresses to classes is known as classful addressing. The allocation of addresses is managed by a central authority, the American Registry for Internet Numbers (ARIN), which you can go to at http://www.arin.net for more information about network numbers.
Five IP address classes are used, as follows:
■ Class A: The Class A address category was designed to support extremely large networks. A Class A address uses only the first octet to indicate the network address. The remaining three octets are used for host addresses. The first bit of a Class A address is always 0; therefore, the lowest number that can be represented is 00000000 (decimal 0), and the highest number that can be represented is 01111111 (decimal 127). However, these two network numbers, 0 and 127, are reserved and cannot be used as a network address. Any address that starts with a value between 1 and 126 in the first octet, then, is a Class A address.
■ Class B: The Class B address category was designed to support the needs of moderateto large-sized networks. A Class B address uses two of the four octets to indicate the network address. The other two octets specify host addresses.
The first 2 bits of the first octet of a Class B address are always binary 10. The remaining 6 bits might be populated with either 1s or 0s. Therefore, the lowest number that can be represented with a Class B address is 10000000 (decimal 128), and the highest number that can be represented is 10111111 (decimal 191). Any address that starts with a value in the range of 128 to 191 in the first octet is a Class B address.
■ Class C: The Class C address category is the most commonly used of the original address classes. This address category was intended to support a lot of small networks.A Class C address begins with binary 110. Therefore, the lowest number that can be represented is 11000000 (decimal 192), and the highest number that can be represented is 11011111 (decimal 223). If an address contains a number in the range of 192 to 223 in the first octet, it is a Class C address.
■ Class A: The Class A address category was designed to support extremely large networks. A Class A address uses only the first octet to indicate the network address. The remaining three octets are used for host addresses. The first bit of a Class A address is always 0; therefore, the lowest number that can be represented is 00000000 (decimal 0), and the highest number that can be represented is 01111111 (decimal 127). However, these two network numbers, 0 and 127, are reserved and cannot be used as a network address. Any address that starts with a value between 1 and 126 in the first octet, then, is a Class A address.
■ Class B: The Class B address category was designed to support the needs of moderateto large-sized networks. A Class B address uses two of the four octets to indicate the network address. The other two octets specify host addresses.
The first 2 bits of the first octet of a Class B address are always binary 10. The remaining 6 bits might be populated with either 1s or 0s. Therefore, the lowest number that can be represented with a Class B address is 10000000 (decimal 128), and the highest number that can be represented is 10111111 (decimal 191). Any address that starts with a value in the range of 128 to 191 in the first octet is a Class B address.
■ Class C: The Class C address category is the most commonly used of the original address classes. This address category was intended to support a lot of small networks.A Class C address begins with binary 110. Therefore, the lowest number that can be represented is 11000000 (decimal 192), and the highest number that can be represented is 11011111 (decimal 223). If an address contains a number in the range of 192 to 223 in the first octet, it is a Class C address.
■ Class D: The Class D address category was created to enable multicasting in an IP address. A multicast address is a unique network address that directs packets with that destination address to predefined groups of IP addresses. Therefore, a singlestation can simultaneously transmit a single stream of datagrams to multiple recipients.
The Class D address category, much like the other address categories, is mathematically constrained. The first 4 bits of a Class D address must be 1110. Therefore, the first octet range for Class D addresses is 11100000 to 11101111, or 224 to 239. An IP address that starts with a value in the range of 224 to 239 in the first octet is a Class D address. As illustrated in Figure 1-30, Class D addresses (multicast addresses) include the following range of network numbers: 224.0.0.0 to 239.255.255.255.
■ Class E: Although a Class E address category has been defined, the Internet Engineering Task Force (IETF) reserves the addresses in this class for its own research. Therefore, no Class E addresses have been released for use in the Internet. The first 4 bits of a Class E address are always set to 1111. Therefore, the first octet range for Class E addresses is 11110000 to 11111111, or 240 to 255.
The Class D address category, much like the other address categories, is mathematically constrained. The first 4 bits of a Class D address must be 1110. Therefore, the first octet range for Class D addresses is 11100000 to 11101111, or 224 to 239. An IP address that starts with a value in the range of 224 to 239 in the first octet is a Class D address. As illustrated in Figure 1-30, Class D addresses (multicast addresses) include the following range of network numbers: 224.0.0.0 to 239.255.255.255.
■ Class E: Although a Class E address category has been defined, the Internet Engineering Task Force (IETF) reserves the addresses in this class for its own research. Therefore, no Class E addresses have been released for use in the Internet. The first 4 bits of a Class E address are always set to 1111. Therefore, the first octet range for Class E addresses is 11110000 to 11111111, or 240 to 255.
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