To accommodate different size networks and aid in classifying these networks, IP addresses are divided into groups called classes. This is known as classful addressing. Each complete 32-bit IP address is broken down into a network part and a host part. A bit or bit sequence at the start of each address determines the class of the address. There are five IP address classes as shown in Figure .

The Class A address was designed to support extremely large networks. Class A IP addresses use only the first octet to indicate the network address. The remaining three octets enumerate host addresses.

The first bit of a Class A address is always 0. With that first bit a 0, the lowest number that can be represented is 00000000, decimal 0. The highest number that can be represented is 01111111, decimal 127. However, these two 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 is a Class A address.

The 127.0.0.0 network is reserved for loopback testing. Routers or local machines can use this address to send packets back to themselves. Therefore, this number cannot be assigned to a network.

The Class B address was designed to support the needs of moderate to large-sized networks. A Class B IP address uses the first two of the four octets to indicate the network address. The other two octets specify host addresses.

The first two bits of the first octet of a Class B address are always 10. The remaining six bits may be populated with either 1s or 0s. Therefore, the lowest number that can be represented with a Class B address is 10000000, decimal 128. 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.

The Class C address space is the most commonly used of the original address classes. This address space was intended to support small networks.

A Class C address begins with binary 110. Therefore, the lowest number that can be represented is 11000000, decimal 192. 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.

The Class D address class 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 single station can simultaneously transmit a single stream of data to multiple recipients.

The Class D address space, much like the other address spaces, is mathematically constrained. The first four 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.

A Class E address has been defined. However, the Internet Engineering Task Force (IETF) reserves these addresses for its own research. Therefore, no Class E addresses have been released for use in the Internet. The first four bits of a Class E address are always set to 1s. Therefore, the first octet range for Class E addresses is 11110000 to 11111111, or 240 to 255.

Figure shows the IP address range of the first octet both in decimal and binary for each IP address class.

Lab Activity IP Addressing Basics This lab is to name the five different classes of IP addresses and to describe the characteristics and use of the different IP address classes.

    

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