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Understanding IPv4 Header Fields

May 17, 2025

Jeremy's IT Lab: IPv4 Header Overview

Introduction

  • Free CCNA course by Jeremy's IT Lab.
  • Subscribe, like, comment, and share videos.
  • Use Anki flashcards for memorization.

Video Focus

  • IPv4 header, part of the OSI model's Layer 3.
  • Video dedicated to understanding IPv4 header fields.
  • Future videos to cover routing and hands-on CLI configuration.

OSI Model Review

  • Layer 4: Encapsulation of data into segments (TCP or UDP).
  • Layer 3: Segments become packets with IP headers.
  • Layer 2: Packets become frames with additional headers/trailers.

IPv4 Header Fields

  1. Version

    • Length: 4 bits
    • Identifies IP version (4 for IPv4, 6 for IPv6).
  2. Internet Header Length (IHL)

    • Length: 4 bits
    • Indicates header length in 4-byte increments.
    • Minimum: 5 (20 bytes), Maximum: 15 (60 bytes).
  3. Differentiated Services Code Point (DSCP)

    • Length: 6 bits
    • Used for QoS, prioritizes delay-sensitive data.
  4. Explicit Congestion Notification (ECN)

    • Length: 2 bits
    • Notifies congestion without dropping packets.
  5. Total Length

    • Length: 16 bits
    • Total length of the packet in bytes.
  6. Identification

    • Length: 16 bits
    • Identifies fragments of the original packet.
  7. Flags

    • Length: 3 bits
    • Bit 0: Reserved, Bit 1: Don't Fragment (DF), Bit 2: More Fragments (MF).
  8. Fragment Offset

    • Length: 13 bits
    • Position of fragment in the original packet.
  9. Time to Live (TTL)

    • Length: 8 bits
    • Prevents infinite loops (decremented at each hop).
  10. Protocol

    • Length: 8 bits
    • Indicates encapsulated Layer 4 protocol (e.g., TCP, UDP).
  11. Header Checksum

    • Length: 16 bits
    • Checks for errors in the IPv4 header.
  12. Source and Destination IP Address

    • Length: 32 bits each
    • Identifies sender and receiver.
  13. Options

    • Variable length: 0 to 320 bits
    • Rarely used.

Practical Application

  • Wireshark used to analyze packet capture.
  • Example of fragmentation with ping command.

Summary

  • Understanding of IPv4 header fields is crucial for CCNA.
  • Use flashcards and supplementary materials for review.
  • Next video will involve CLI configurations.

Quiz Recap

  1. Fixed value of version field in IPv4 header: 0100.
  2. Field causing packet drop at 0: TTL.
  3. Error detection in encapsulated data: via encapsulated protocol.
  4. Variable length field in IPv4: Options.
  5. Bit set on all fragments except last: More Fragments.

Conclusion

  • Use additional resources and tools like Anki and Wireshark.
  • Stay tuned for hands-on routing configurations in upcoming videos.
  • Support channel via subscriptions and shares.