Address Classes

The Internet community originally defined five address classes to accommodate networks of varying sizes. Microsoft TCP/IP supports class A, B, and C addresses assigned to hosts. The class of address defines which bits are used for the network ID and which bits are used for the host ID. It also defines the possible number of networks and the number of hosts per network.

Class A

Class A addresses are assigned to networks with a very large number of hosts. The high-order bit in a class A address is always set to zero. The next seven bits (completing the first octet) complete the network ID. The remaining 24 bits (the last three octets) represent the host ID. This allows for 126 networks and 16,777,214 hosts per network. Figure 1.4 illustrates the structure of class A addresses.

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Figure 1.4 Class A IP Addresses

Class B

Class B addresses are assigned to medium-sized to large-sized networks. The two high-order bits in a class B address are always set to binary 1 0. The next 14 bits (completing the first two octets) complete the network ID. The remaining 16 bits (last two octets) represent the host ID. This allows for 16,384 networks and 65,534 hosts per network. Figure 1.5 illustrates the structure of class B addresses.

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Figure 1.5 Class B IP Addresses

Class C

Class C addresses are used for small networks. The three high-order bits in a class C address are always set to binary 1 1 0. The next 21 bits (completing the first three octets) complete the network ID. The remaining 8 bits (last octet) represent the host ID. This allows for 2,097,152 networks and 254 hosts per network. Figure 1.6 illustrates the structure of class C addresses.

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Figure 1.6 Class C IP Addresses

Class D

Class D addresses are reserved for IP multicast addresses. The four high-order bits in a class D address are always set to binary 1 1 1 0. The remaining bits are for the address that interested hosts recognize. Microsoft supports class D addresses for applications to multicast data to multicast-capable hosts on an internetwork.

Class E

Class E is an experimental address that is reserved for future use. The high-order bits in a class E address are set to 1111.

Table 1.11 is a summary of address classes A, B, and C that can be used for host IP addresses.

Table 1.11 IP Address Class Summary

Network ID Guidelines

The network ID identifies the TCP/IP hosts that are located on the same physical network. All hosts on the same physical network must be assigned the same network ID to communicate with each other.

Follow these guidelines when assigning a network ID:

• The network ID must be unique to the IP internetwork. If you plan on having a direct routed connection to the public Internet, the network ID must be unique to the Internet. If you do not plan on connecting to the public Internet, the local network ID must be unique to your private internetwork.
• The network ID cannot begin with the number 127. The number 127 in a class A address is reserved for internal loopback functions.
• All bits within the network ID cannot be set to 1. All 1's in the network ID are reserved for use as an IP broadcast address.
• All bits within the network ID cannot be set to 0. All 0's in the network ID are used to denote a specific host on the local network and are not routed.

Table 1.12 lists the valid ranges of network IDs based on the IP address classes. To denote IP network IDs, the host bits are all set to 0. Note that even though expressed in dotted decimal notation, the network ID is not an IP address.

Table 1.12 Class Ranges of Network IDs

Host ID Guidelines

The host ID identifies a TCP/IP host within a network. The combination of IP network ID and IP host ID is an IP address.

Follow these guidelines when assigning a host ID:

• The host ID must be unique to the network ID.
• All bits within the host ID cannot be set to 1 because this host ID is reserved as a broadcast address to send a packet to all hosts on a network.
• All bits in the host ID cannot be set to 0 because this host ID is reserved to denote the IP network ID.

Table 1.13 lists the valid ranges of host IDs based on the IP address classes.

Table 1.13 Class Ranges of Host IDs

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