Class C addresses

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A Class C address consists of a 24-bit network address and an 8-bit local host address.

The first three bits in the network address indicate the network class, leaving 21 bits for the actual network address. Therefore, there are 2,097,152 possible network addresses and 256 possible local host addresses. In a Class C address, the highest order bits are set to 1-1-0.

Figure 1. Class C address
Class C address
This figure shows a typical class C address structure. The first 24 bits contain the network address (the three highest order bits will always be 1-1-0). The remaining 8 bits contain the local host address.

In other words, the first octet of a Class C address is in the range 192 to 223.

When deciding which network address class to use, you must consider how many local hosts there will be on the network and how many subnetworks will be in the organization. If the organization is small and the network will have fewer than 256 hosts, a Class C address is probably sufficient. If the organization is large, then a Class B or Class A address might be more appropriate.

Note: Class D (1-1-1-0 in the highest order bits) addresses provide for multicast addresses and are supported by UDP/IP under this operating system.

Machines read addresses in binary code. The conventional notation for Internet host addresses is the dotted decimal, which divides the 32-bit address into four 8-bit fields. The following binary value:

0001010 00000010 00000000 00110100

can be expressed as:

010.002.000.052 or 10.2.0.52

where the value of each field is specified as a decimal number and the fields are separated by periods.

Note: The hostent command does recognize the following addresses: .08, .008, .09, and .009. Addresses with leading zeros are interpreted as octal, and numerals in octal cannot contain 8s or 9s.

TCP/IP requires a unique Internet address for each network interface (adapter) on a network. These addresses are determined by entries in the configuration database, which must agree with entries in the /etc/hosts file or the named database if the network is using a name server.