IPv4 Header Options Field Maximum Length

H12-811 By islever Editorial Last updated

IP Addressing and Configuration

Short answer

A The maximum length of the IPv4 Options field is 40 bytes, derived from the 60-byte maximum total header size minus the 20-byte fixed header.

Question

The last option field (option) of an IPv4 packet is a length-variable and optional field. What is the
maximum length of this field?
  1. 40B Correct Answer
  2. 10B
  3. 20B
  4. 60B

Key Takeaways

Tests the understanding that the IPv4 header maxes out at 60 bytes total, meaning the optional portion cannot exceed 40 bytes after accounting for the fixed 20-byte header.

This question asks for the maximum length of the IPv4 packet's optional field. The correct answer is 40 bytes, based on the 60-byte maximum header limit.

Common mistake: Choosing D (60B) by confusing the total maximum header size with the specific size of the optional field alone.

Expert Analysis

IPv4 Header Structure and Constraints

The IPv4 header is composed of a fixed portion of 20 bytes and an optional Options field. The total header length is determined by the Internet Header Length (IHL) field, which counts the number of 32-bit words in the header. Since the minimum IHL value is 5 (representing 20 bytes), the header can be extended by adding options.

Calculating the Maximum Options Length

According to the IPv4 specification (RFC 791), the maximum total length of the IPv4 header is 60 bytes. This limit arises because the IHL field is only 4 bits long, allowing for a maximum value of 15. Multiplying 15 by 4 bytes per word yields 60 bytes. To find the maximum length of the Options field, we subtract the fixed header size from the maximum total header size:

  • Maximum Total Header: 60 bytes
  • Fixed Header: 20 bytes
  • Maximum Options Field: 40 bytes

Evaluating the Options

  • A. 40B: Correct. This matches the calculated maximum length derived from the 60-byte header limit minus the 20-byte fixed header.
  • B. 10B, C. 20B, D. 60B: Incorrect. These values do not align with the standard IPv4 header constraints. Specifically, 60B represents the total header size, not just the options portion.

The question tests knowledge of the rigid structural limits of the IPv4 protocol header.

Official Reference

https://www.rfc-editor.org/rfc/rfc791

This link opens the official vendor documentation in a new tab.

Exam Strategy

When answering IPv4 structure questions, always remember the 4-bit IHL field limit. Calculate the maximum total header size (15 * 4 = 60 bytes) first, then subtract the known fixed components (like the 20-byte base header) to isolate the variable parts like Options or Padding.

Frequently Asked Questions

Why is the IPv4 header limited to 60 bytes?

The IHL (Internet Header Length) field is 4 bits, allowing a maximum value of 15. Since each unit is 4 bytes, 15 * 4 = 60 bytes maximum.

What happens if the Options field exceeds 40 bytes?

It is impossible under the IPv4 standard because the total header cannot exceed 60 bytes. If more data is needed, it must go into the payload, not the header.

More H12-811 FAQ →

Discussion 7 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 7

  1. Chleong 👍 2 Answered A
    Option length 0-40
  2. IBB90704 👍 1
    La opcion es la A Option field 0- 40 bytes The maximum length of the Options field is 40 bytes.
  3. eeadbff 👍 1 Answered A
    40B is for option field. datagram header can be up to 60B due to option field
  4. Fmoura17 👍 1 Answered A
    Answer: A. 40B Max header = 60B minus Fixed header = 20B Options = 40B ✅
  5. Chleong 👍 1 Answered D
    Answer is correct
Show 2 more comments
  1. Creyes1495 👍 1 Answered A
    A. 40B, Pag. 131
  2. HCIA 👍 1 Answered A
    40 bytes

Reader comments & corrections

No reader corrections yet — spotted an error or have a note? Share it below.

Log in to comment, report an error, or add a note about this question.

Entering OSPF Area View in Huawei VRP

S-grade

IEEE 802.11ac Frequency Band Support

S-grade

Huawei AR G3 AAA Authentication Modes

S-grade

Switch Port Handling of Untagged Frames and PVID

S-grade

← Back to H12-811 Study Guide