Internet Protocol Summary - Facts about IP

By Bradley Mitchell, About.com
Name: Internet Protocol - abbreviated "IP"
Description: IP is used by many higher level network protocols, principally TCP and UDP. Many Internet software applications including Web browsers, FTP clients, and email programs, rely on Internet Protocol.
OSI Model: Network layer (Layer 3)
Datagram Format: A base header 20 bytes (5 "longwords") in length, with the option for expanded header options, followed by data.

Header:

Word 1 -
• Version - 4 bits
• Header Length (in longwords) - 4 bits
• Type of Service / Differentiated Services Code Point (DSCP) - 8 bits
• Datagram Length (in bytes) - 16 bits
Word 2 -
• ID Number - 16 bits
• Fragmentation Flags - 3 bits
• Fragmentation Offset - 13 bits
Word 3 -
• Time to Live - 8 bits
• Transport Protocol - 8 bits
• Header Checksum - 16 bits
Word 4 -
• Source IP Address - 32 bits
Word 5 -
• Destination IP Address - 32 bits
Variable length fields -
• Options
• Padding
Payload: IP datagram payloads can be of variable length. The minimum size of an IP datagram is 28 bytes, using the minimum 20 bytes of header information, followed by the minimum of 8 bytes of data. The maximum size of an IP datagram payload is 65,535 bytes minus the header size.
Footer: Internet Protocol does not use its own datagram footer.

source :http://compnetworking.about.com