IPv4 Header Format

The IPv4 header is 20 to 60 bytes (20-byte base header plus up to 40 bytes of options).

Field	Size	Purpose
Version	4 bits	IP version (4)
IHL	4 bits	Header length in 32-bit words
Type of Service	8 bits	DSCP/ECN; QoS
Total Length	16 bits	Header + data, max 65,535 bytes
Identification	16 bits	Identifies fragments of one datagram
Flags	3 bits	Reserved, DF, MF
Fragment Offset	13 bits	Position of fragment
TTL	8 bits	Hop limit
Protocol	8 bits	Upper layer (TCP=6, UDP=17, ICMP=1)
Header Checksum	16 bits	Header error detection
Source / Destination IP	32 bits each	Addresses

Functions of key fields

• TTL: set by the source and decremented by each router; when it reaches 0 the packet is discarded (ICMP Time Exceeded). Prevents infinite looping.
• Header Checksum: 16-bit one's complement checksum over the header only. Recomputed at every router since TTL changes. Packets with a bad checksum are dropped.
• Flags: 3 bits: bit 0 reserved; DF (Don't Fragment): 1 means do not fragment; MF (More Fragments): 1 means more fragments follow, 0 means last fragment.
• Fragmentation Offset: gives the position of the fragment's data in the original datagram, in units of 8 bytes, allowing the receiver to reassemble fragments in order.