CCNA | IPv4 Header | 10

Introduction to IPv4 Header

• Overview of the IP (Internet Protocol) header structure.

• Focus on the IPv4 header encapsulation used for TCP or UDP segments.

• Purpose of the video: Delve into the fields of the IPv4 header.

Structure of Protocol Data Units (PDUs)

• PDUs are the data units defined by different layers of the OSI model:

  • Layer 4: Segment (data + TCP/UDP header)

  • Layer 3: Packet (segment + IPv4 header)

  • Layer 2: Frame (packet + Layer 2 header/trailer)

• Emphasis on Layer 3 and its importance for routing.

Fields of IPv4 Header

• Examination of various fields within the IPv4 header:

  • Version Field

    • Length: 4 bits (0-15)

    • Indicates the IP protocol version; for IPv4, it holds the value 4.

    • Other possibilities include IPv6 (value 6).

  • Internet Header Length (IHL)

    • Length: 4 bits

    • Specifies the header's length in 4-byte increments.

    • Minimum value is 5 (20 bytes), maximum is 15 (60 bytes).

  • Differentiated Services Code Point (DSCP)

    • Length: 6 bits

    • Used for Quality of Service (QoS) to prioritize certain data types.

  • Explicit Congestion Notification (ECN)

    • Length: 2 bits

    • Signifies network congestion without dropping packets.

  • Total Length

    • Length: 16 bits (up to 65,535 bytes)

    • Specifies the total packet length including the header and encapsulated data.

  • Identification Field

    • Length: 16 bits

    • Differentiates between packets to reassemble fragments when necessary.

  • Flags Field

    • Length: 3 bits

    • Controls fragmentation; includes 'Don't Fragment' and 'More Fragments' indicators.

  • Fragment Offset

    • Length: 13 bits

    • Indicates the position of a fragment within the original packet.

  • Time To Live (TTL)

    • Length: 8 bits

    • Prevents looping; decreases value by 1 with each hop through a router.

  • Protocol Field

    • Length: 8 bits

    • Indicates the encapsulated Layer 4 protocol (e.g., TCP = 6, UDP = 17, ICMP = 1).

  • Header Checksum

    • Length: 16 bits

    • Error-checking; helps identify corruption in the header during transmission.

  • Source IP Address

    • Length: 32 bits

    • Represents the sender's IPv4 address.

  • Destination IP Address

    • Length: 32 bits

    • Represents the receiver's IPv4 address.

  • Options Field

    • Variable length (0 to 40 bytes)

    • Rarely used; present if IHL > 5.

Practical Application of IPv4 Headers Using Wireshark

• Overview of packet capture using Wireshark:

  • Captured a ping request from two routers.

  • Breakdown of Ethernet header, IPv4 header, and ICMP payload in Wireshark.

  • Detailed examination of the IP header fields displayed in a packet capture.

• Explanation of packets that exceed MTU leading to fragmentation and how fragmentation is indicated in the IPv4 header.

Review and Summary

• Recap of the IPv4 header fields discussed.

• Importance of understanding header structures for practical router configurations.

• Mention of upcoming videos focusing on hands-on CLI configurations.

Conclusion

• Prepare for continued learning through practice quizzes and upcoming practical sessions in the course.