IPv4 is the original version of IP addressing on the internet. An IP address is commonly written in dotted decimal notation and the address consists of four numbers which are 8 bits (bytes) each. Each of the four numbers in the IP address is separated by a dot as it is written. An example of an IPv4 address is 192.168.1.1. IPv4 provides for up to 4,294,967,295 unique addresses for a total address length of 32 bits or 4 bytes. Some of the addresses that IPv4 provides are reserved for private networks such as the one in the example above. The reason for creating the new IPv6 IP addressing scheme is because IPv4 does not provide enough IP addresses for the internet to be used by everyone.
RFC 791 describes IPv4.
The page called Internet Protocol describes the IP message format in detail.