The structure of IP packets facilitates their efficient transmission over the Internet. Every IP packet consists of two parts: a header and a body. The header contains the packet’s origin and destination addresses, as well as information about the type of data contained in the body of the packet. The IP protocol prescribes the size of the header and the sequence of information fields in it. For example, in IP version 4, which is still commonly used in North America, the 13th through 16th bytes in the header always contain the IP address of the source of the packet, and the 17th through 20th bytes always contain the destination address. This well-defined structure makes it easy for computers to decide where to send each packet that arrive.