In order for IP addresses to remain unique, somebody has to assign them and keep track of them. For the Internet, this is the Internet Assigned Numbers Authority (IANA) that assigns blocks of IP addresses to organizations such as ISPs. For your company network, it would be your network administrator who has a block of IP addresses that he assigns to every computer in the company. For your Internet access, it would be your ISP that has a pool of IP’s that they assign to their customers. For your home network, if you set up a small home network, you would assign an IP address to your computers. For the specific purpose of home networking, IANA has reserved several blocks of numbers as private IP numbers that can be used in private networks only but not on the Internet. These blocks are:
|10.0.0.0 – 10.255.255.255|
|172.16.0.0 – 172.31.255.255|
|192.168.0.0 – 192.168.255.255|
There is a reason that there are three blocks of numbers, and not just one big block. IP addresses are separated into 3 classes, simply called Class A, B, and C.
Class A addresses are used for systems with a small number of networks, but a large number of computers (hosts). They use only the first byte/octet of the IP as network ID, and the remaining 3 bytes as host ID.
Class B addresses are used for systems with a roughly equal number of hosts and networks, They use the first two bytes/octets of the IP as network ID, and the remaining two bytes as host ID.
Class C addresses are used for systems with a large number of networks, but a small number of computers (hosts) on each network. They use the first 3 bytes/octets as network ID, and the remaining byte as host ID.
In order to identify which class is used, something called a subnet mask is used. If a byte is used for network ID, that byte of the subnet mask will be 255. If a byte is not used for the network ID but for the host ID instead, that byte of the subnet mask will be 0. The following table should make this a little clearer:
|Class||IP Example||Network ID||Host ID||Subnet Mask|