A numeric code that identifies a particular computer on the Internet . Just as a street address identifies the location of your home or office , every computer or network on the Internet has a unique address, too. You register your address with InterNIC as both a name , which is referred to as the domain name , and a number ( ) , which is generally referred to as the IP address or IP number. Most likely, the InterNIC will assign you a Class C address, which consists of 255 unique IP numbers for you to assign to your employees.
If you need more than 255 IP address, you can apply for a Class B address, which will give you over 65,000 unique IP addresses. Class A addresses are for very large companies. Both Class A and Class B addresses are very hard, if not impossible, to get. Usually, companies will get multiple Class C addresses.

Internet address
The address can be split into a network number (or network address) and a host number unique to each host on the network and sometimes also a subnet address. The way the address is split depends on its "class", A, B or C as determined by the high address bits:

Class A - high bit 0, 7-bit network number, 24-bit host number. n1.a.a.a 0 <= n1 <= 127

Class B - high 2 bits 10, 14-bit network number, 16-bit host number. n1.n2.a.a 128 <= n1 <= 191

Class C - high 3 bits 110, 21-bit network number, 8-bit host number. n1.n2.n3.a 192 <= n1 <= 223

Network number: The network portion of an IP address. For a class A network, the network address is the first byte of the IP address. For a class B network, the network address is the first two bytes of the IP address. For a class C network, the network address is the first three bytes of the IP address. In each case, the remainder is the host address. In the Internet, assigned network addresses are globally unique.

Host number: The host part of an Internet address. The rest is the network number.

Subnet address : The subnet portion of an IP address. In a subnetted network, the host portion of an IP address is split into a subnet portion and a host portion using an address mask (the subnet mask).

Address mask : A bit mask used to identify which bits in an IP address correspond to the network address and subnet portions of the address. This mask is often referred to as the subnet mask because the network portion of the address can be determined by the class inherent in an IP address. The address mask has ones in positions corresponding to the network and subnet numbers and zeros in the host number positions.

Details about Internet Addresses: Subnets and Broadcasting

The assumption is that most of the networks will be small. So they set up three different ranges of address. Addresses beginning with 1 to 126 use only the first octet for the network number. The other three octets are available for the host number. Thus 24 bits are available for hosts. These numbers are used for large networks. But there can only be 126 of these very big networks. The Arpanet is one, and there are a few large commercial networks. But few normal organizations get one of these "class A" addresses. For normal large organizations, "class B" addresses are used. Class B addresses use the first two octets for the network number. Thus network numbers are 128.1 through 191.254. (We avoid 0 and 255, for reasons that we see below. We also avoid addresses beginning with 127, because that is used by some systems for special purposes.) The last two octets are available for host addesses, giving 16 bits of host address. This allows for 64516 computers, which should be enough for most organizations. (It is possible to get more than one class B address, if you run out.) Finally, class C addresses use three octets, in the range 192.1.1 to 223.254.254. These allow only 254 hosts on each network, but there can be lots of these networks. Addresses above 223 are reserved for future use, as