Computer Networks | Internet Protocol | Datagram Format
IP is a connection-less and unreliable protocol. IP packets may be delivered out of order or may cause congestion in the network. For ensuring reliability, IP should be paired with Transmission Control Protocol (a reliable protocol). The IP datagram format may be thought of as consisting of two parts - header (20 bytes - 60 bytes) and data (20 bytes - 65535 bytes).
The header may be thought of as having 6 rows:
Row 1 : VER (4 bits) | HLEN (4 bits) | Service Type (8 bits) | Total length (16 bits)
Row 2 : Identification (16 bits) | Flags (3 bits) | Fragmentation Offset (13 bits)
Row 3 : TTL (8 bits) | Protocol (8 bits) | Header Checksum (16 bits)
Row 4 : Source IP Address
Row 5 : Destination IP Address
Row 6 : Options (0 bytes - 40 bytes)