An IP address is the "Internet Protocol" address which identifies your computer to the rest of the Internet. It's a bit like a telephone number on the phone network.
Your IP address as shown on this page is what every single website you visit will see.
The key to understanding IP, and all of the issues related to IP, is knowing what a routing table looks like and the effects each IP topic has on the entries in a routing table. To begin with, let's review the basics. IP addresses are 32 bit numbers, most commonly represented in dotted decimal notation (xxx.xxx.xxx.xxx). Each decimal number represents eight bits of binary data, and therefore can have a decimal value between 0 and 255. IP addresses most commonly come as class A, B, or C. It's the value of the first number of the IP address that determines the class to which a given IP address belongs. Class D addresses are used for multi-cast applications.
IPv4 addresses are32-bits in size, which limits the address space to 4294967296 (232) IP addresses. IPv4 reserves some addresses for special purposes such as private networks (~18 million addresses) or multicast addresses (~270 million addresses).
The address size was increased from 32 bits in IPv4 to 128 bits or 16 octets in IPv6, providing up to 2128 (approximately 3.403×1038) addresses. These figures are deemed as sufficient for the future.