Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 43
Class C Address

For those who answered “25 percent,” this is correct. With the first two bits reserved, this leaves 30 bits for address assignment. 230 is 25 percent of the available address space. Class C takes the form of <network number .network number.network number.host>, bytes 0, 1, 2, and 3. Class C addresses use the first 3 out of 4 bytes of the address for the network number and the last field for the host number. This allows lots of networks with a fewer hosts per network. A Class C address is identified by the first 3 bits of the first field. If the first and second bits are 1s and the third bit is a 0, this will identify a Class C address (110xxxxx). Since the first 3 bits in the first field will always be a 110xxxxx, the allowable network range is 192–223 in the first field. This allows for 2,097,152 (2ⁿ21) possible network addresses. All of the bits in the second and third fields are allowed to be used (including all 0s and 1s). Therefore, the whole allowable range for Class C network addresses is 192 to 223 (in the first field), 0 to 255 (in the second field), and 0 to 255 (in the third field). The last field will range from 1 to 254 for host assignment. This allows 2,097,152 network numbers, each capable of supporting 254 hosts (all 0s and all 1s are still reserved no matter what type of routing and addressing you are using). No host can be assigned a 0 or all 1s as its address. Class C addresses allow only 254 hosts per network number. Notice that the largest number in the first field may go up to 223. Any number over 223 in the first field will indicate a Class D address. Class D addresses are reserved as multicast addresses.

Class C addresses are the most commonly assigned by the NIC. Class B addresses have been exhausted. Therefore, ISPs and regional Internet Registries are assigning Class C and Class A (with subnets).

Class C Address

Okay, yep, one more question: “How much of the address space is defined by Class C’s bit reservation of 110?”