Coursify

Maximum Header Size of an IP Packet (IPv4 vs IPv6)

Maximum Header Size of an IP Packet (IPv4 vs IPv6)

Verified Sources
Nov 4, 2026

The question asks: “What is the maximum header size of an IP packet?” In most networking exam contexts, this refers to IPv4, whose header size can vary because it may include options. The IPv4 header includes an IHL (Internet Header Length) field that specifies the header length in 32-bit words, allowing a maximum IPv4 header size of 60 bytes. This corresponds to 15 words × 4 bytes/word.2

Key point: “Maximum header size” for IPv4 is 60 bytes, not the minimum (20 bytes).2

Core concepts (quick definitions)

  • IPv4
  • IHL
  • IP options
  • IPv6

Footnotes

  1. RFC 791 - Internet Protocol - Defines the IPv4 header format and IHL field as header length in 32-bit words. ↩ ↩2

  2. IPv4 Header - Internet Protocol (IHL max implies 60 bytes) - Explains that IHL is 4 bits and maximum corresponds to 60 bytes (15 × 4). ↩ ↩2

IPv4 Header Length (IHL) Explained

To avoid confusion with IPv6: IPv6’s base header is fixed at 40 bytes, but IPv6 uses extension headers that add additional bytes beyond the base header. Therefore, if an exam says “maximum header size” without mentioning IPv6 extension headers, the standard intended answer is typically the IPv4 maximum header size = 60 bytes.3

Footnotes

  1. RFC 8200 - Internet Protocol, Version 6 (IPv6) Specification - Specifies IPv6 base header size (40 bytes) and the role of extension headers. ↩ ↩2

  2. RFC 791 - Internet Protocol - Defines the IPv4 header format and IHL field as header length in 32-bit words. ↩

  3. IPv4 Header - Internet Protocol (IHL max implies 60 bytes) - Explains that IHL is 4 bits and maximum corresponds to 60 bytes (15 × 4). ↩

How the “Max Header Size” is determined (common exam intent)

Assume IPv4

Step 1

Look for IPv4 header’s variable length due to options."

Use IHL field rule

Step 2

IHL is in 32-bit words and tops out at 15 words."

Convert words to bytes

Step 3

15×4=6015 \times 4 = 60 bytes maximum IPv4 header."

Compute maximum IPv4 header size from IHL

  1. 1
    Step 1

    IPv4 IHL is measured in 32-bit (4-byte) words.

    Footnotes

    1. RFC 791 - Internet Protocol - Defines the IPv4 header format and IHL field as header length in 32-bit words. ↩

  2. 2
    Step 2

    The IHL field is 4 bits, allowing a maximum value of 15 (words).

    Footnotes

    1. IPv4 Header - Internet Protocol (IHL max implies 60 bytes) - Explains that IHL is 4 bits and maximum corresponds to 60 bytes (15 × 4). ↩

  3. 3
    Step 3

    Maximum header bytes =15×4=60= 15 \times 4 = 60 bytes.

  4. 4
    Step 4

    Therefore, among the options, the correct answer is (ii) 64 bytes is not correct; (iv) 60 bytes is correct.

IPv4 vs IPv6 header sizes (base header)

Most “maximum header size” exam questions refer to IPv4’s variable header (options included).

Exam Strategy

If the question says “maximum header size of an IP packet” without mentioning IPv6 extension headers, it almost always targets IPv4 IHL maximum = 60 bytes.2

Footnotes

  1. RFC 791 - Internet Protocol - Defines the IPv4 header format and IHL field as header length in 32-bit words. ↩

  2. IPv4 Header - Internet Protocol (IHL max implies 60 bytes) - Explains that IHL is 4 bits and maximum corresponds to 60 bytes (15 × 4). ↩

Don’t mix “base header” with “all headers”

IPv6 has a fixed 40-byte base header, but extension headers can add more bytes. So “maximum header size” depends on whether the test means base header only or includes extension headers.

Footnotes

  1. RFC 8200 - Internet Protocol, Version 6 (IPv6) Specification - Specifies IPv6 base header size (40 bytes) and the role of extension headers. ↩

Common confusion points

Knowledge Check

Question 1 of 1
Q1Single choice

What is the maximum header size of an IP packet (typical IPv4 interpretation)?

Explore Related Topics

1

TCP/IP Stack vs OSI Transport Layer: Correct Mapping

2

IPv4 Header Format and Key Fragmentation/Integrity Fields (TTL, Header Checksum, Flags, Fragment Offset)

3

Subnetting Practice: Valid Host for 192.168.4.32/68 (IPv4)

Browse all research articles