Protocol Tutorial

IPv4 Header Format

In-depth guide to IPv4 header structure: version, IHL, TOS, total length, identification, flags, fragment offset, TTL, protocol, header checksum, source address, and destination address.

Protocol Overview

Internet Protocol version 4 (RFC 791)

Header Size: 20 bytes (160 bits)
Fields: 13
Endianness: big
Spec: RFC 791

Header Fields

FieldOffsetSizeTypeDescription
Version
version
0 bits4 bitsuint8IP version (always 4 for IPv4)
IHL
ihl
4 bits4 bitsuint8Internet Header Length in 32-bit words (minimum 5)
DSCP
dscp
8 bits6 bitsuint8binaryDifferentiated Services Code Point (QoS)
ECN
ecn
14 bits2 bitsuint8binaryExplicit Congestion Notification
Total Length
total_length
16 bits16 bitsuint16Total length of IP packet in bytes (header + data)
Identification
identification
32 bits16 bitsuint16hexUnique identifier for fragmentation reassembly
Flags
flags
48 bits3 bitsuint8binaryControl flags (Reserved, DF, MF)
Fragment Offset
fragment_offset
51 bits13 bitsuint16Position of fragment in original datagram (in 8-byte blocks)
TTL
ttl
64 bits8 bitsuint8Time To Live (hop limit)
Protocol
protocol
72 bits8 bitsuint8Protocol used in data portion
Known values: 1=ICMP, 2=IGMP, 6=TCP, 17=UDP, +5 more
Header Checksum
header_checksum
80 bits16 bitsuint16hexChecksum of IP header only
Source IP
source_ip
96 bits32 bitsuint32hexSource IP address
Destination IP
dest_ip
128 bits32 bitsuint32hexDestination IP address

Common IPv4 Mistakes

  • 1Confusing IHL (32-bit words) with header length in bytes (multiply IHL by 4)
  • 2Forgetting that Total Length includes both header and payload
  • 3Misreading the Flags field: DF (Don't Fragment) and MF (More Fragments) are critical bits
  • 4Not understanding that Fragment Offset is in 8-byte blocks, not bytes
  • 5Assuming TTL is in seconds (it's actually hop count in modern implementations)

Try It Yourself

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Related Resources

IPv4 Protocol Visualizer - Parse & Analyze IPv4 PacketsIPv4 Basic Packet Example - ByteLensIPv4 vs IPv6 - Key Differences and Migration GuideIP Address Protocol - What is IP? IPv4 vs IPv6 Explained

Quick Facts

ProtocolIPv4
Header Size20 bytes
Fields13
Endiannessbig
SpecificationRFC 791