An IP address is a number coded on 32 bits (32 "0" or "1") which represents a unique ID for a host on an IP Network. IP networks are the most popular kind of networks. The most famous IP network being Internet !

The IP address is often represented with 4 bytes (or octets) composed by 8 bits each. Each byte is written in decimal notation, in the range 0 to 255, separated by dots. That's what we call the "dotted decimal" notation.

For example, the IP address: 192. 10. 15. 1
Is in fact in binary : 11000000.00001010.00001111.00000001 (see "How to convert decimal to binary ?")

IP addresses must be unique on a same IP network, however, they might be redundant on separate ones.
On Internet, which is a Word Wild IP network, all IP addresses must be unique. It is a little bit like a postal address: if you want to receive your mail, your house number, street and postcode must be unique.
The organisation in charge of allocating IP addresses on Internet is the Internet Assigned Numbers Authority (IANA).
Note that some address ranges, defined in RFC 1597 and RFC 1918 are reserved for private network and are not routed on Internet (namely 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16).

IP adresses are composed of two parts: the "Network" and the "Host" (also called "Node").The Class of the IP address determine which part of the address codes the Network and which part codes the Host (see "What is an IP Class ?").