More About Network Addresses

Each network running TCP/IP must have a unique network number, and every machine on it must have a unique IP address. It is important to understand how IP addresses are constructed before you register your network and obtain its network number from the InterNIC addressing authority.

The IP address is a 32-bit number that uniquely identifies a network interface on a machine. An IP address is typically written in decimal digits, formatted as four 8-bit fields separated by periods; for example, 129.144.50.156. Each 8-bit field represents a byte of the IP address. This form of representing the bytes of an IP address is often referred to as the dotted-decimal format.

The bytes of an IP address are further classified into two parts: the network part and the host (computer) part. For example, in a typical IP address, such as 129.144.50.56, the first two fields are the network identifier, and the last two fields are the host identifier.

Network Identifier -- This part specifies the unique number assigned to your network. It also identifies the class of network assigned (see Network Classes, below).
Host Part -- This is the part of the IP address that you assign to each host (computer, printer, gateway, and so forth). It uniquely identifies this machine on your network. Note that for each host on your network, the network part of the address will be the same, but the host part must be different.
Subnetwork Number (Optional) -- Local networks with large numbers of hosts are sometimes divided into subnetworks. If you choose to divide your network into subnetworks, you need to assign a subnetwork number for the subnetwork in the netmasks table. You can maximize the efficiency of the IP address space by using some of the bits from the host number part of the IP address as a network identifier. When used as a network identifier, the specified part of the address becomes the subnetwork number. You create a subnetwork number by using a netmask, which is a bit mask that selects the network and subnetwork parts of an IP address.

Note: Network names, aliases, and IP addresses are stored in the networks table on the management server. Netmask information is stored in the netmasks table. If you plan on creating subnetworks within a network defined in the networks table, you must also define a netmask value for the network in the netmasks table.

Network Classes

The first step in planning for IP addressing on your network is to determine which network class is appropriate for your network. After you have done this, you can take the crucial second step: obtain the network number from the InterNIC addressing authority.

Currently there are three classes of TCP/IP networks: class A, class B, and class C, with class A being the most rare but supporting the largest number of hosts. Each class uses the 32-bit IP address space differently, providing more or fewer bits for the network part of the address.

Class A Network Numbers -- A class A network number uses the first eight bits of the IP address as its "network part." The remaining 24 bits comprise the host part of the IP address.
The values assigned to the first byte of class A network numbers fall within the range 0-127. Consider the IP address 75.4.10.4. The value 75 in the first byte indicates that the host is on a class A network. The remaining bytes, 4.10.4, establish the host address. The InterNIC assigns only the first byte of a class A number. Use of the remaining three bytes is left to the discretion of the owner of the network number. Only 127 class A networks can exist. Each one of these numbers can accommodate up to 16,777,214 hosts.
Class B Network Numbers -- A class B network number uses 16 bits for the network number and 16 bits for host numbers. The first byte of a class B network number is in the range 128-191. In the number 129.144.50.56, the first two bytes, 129.144, are assigned by the InterNIC, and comprise the network address. The last two bytes, 50.56, make up the host address, and are assigned at the discretion of the owner of the network number.
Class B is typically assigned to organizations with many hosts on their networks.
Class C Network Numbers -- Class C network numbers use 24 bits for the network number and 8 bits for host numbers. Class C network numbers are appropriate for networks with few hosts-the maximum being 254. A class C network number occupies the first three bytes of an IP address. Only the fourth byte is assigned at the discretion of the network owners.
The first byte of a class C network number covers the range 192-223. The second and third each cover the range 1-255. A typical class C address might be 192.5.2.5. The first three bytes, 192.5.2, form the network number, and the final byte, 5, is the host number.