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IPv6 base header: every field and its purpose

IPv6: addresses and packet format

TEE · shared foundations and Units 3–4 · current syllabus 702AI0C023

Definition. The IPv6 base header is a fixed 40-byte structure containing addressing and forwarding-control fields, followed by optional extension headers and upper-layer data.

Simple meaning. Base header fixed 40 bytes; extra information extension headers mein.

Swipe across the diagram to read its labels ↔

Diagram: IPv6 base header · each horizontal row is 32 bits wide. Labels: IPv6 base header · each horizontal row is 32 bits wide | IPv6 base header · each horizontal row is 32 bits wide | Version | 4 | Traffic Class | 8 | Flow Label · 20 | Payload Length · 16 | Next Header | 8 | Hop Limit | 8 | Source address · 128 bits (four rows) | Destination address · 128 bits (four rows)

Understand it step by step

1. Read the header from its control fields to its two 128-bit addresses. Four plus eight plus twenty bits fills the first 32-bit row.
2. The first two rows together contribute 64 bits; source and destination contribute 256 bits. Total=320 bits=40 bytes.
3. Payload Length counts bytes after the base header, including extension headers when present. It does not include the base 40 bytes.
4. Next Header permits chaining: a base header may point to an extension, which points to another extension or TCP/UDP.
5. The Hop Limit is reduced by forwarding nodes to prevent indefinite circulation. IPv6 has no base-header checksum, and routers do not fragment IPv6 packets.

Practice question. Draw the IPv6 base header and explain its fields. [3 practice marks]

Answer

Swipe across the diagram to read its labels ↔

Diagram: IPv6 base header · each horizontal row is 32 bits wide. Labels: IPv6 base header · each horizontal row is 32 bits wide | IPv6 base header · each horizontal row is 32 bits wide | Version | 4 | Traffic Class | 8 | Flow Label · 20 | Payload Length · 16 | Next Header | 8 | Hop Limit | 8 | Source address · 128 bits (four rows) | Destination address · 128 bits (four rows)

Field	Bits	Purpose
Version	4	Value 6
Traffic Class	8	Traffic treatment/congestion-related marking
Flow Label	20	Identifies a flow for appropriate handling
Payload Length	16	Bytes after the base header; includes extension headers when present
Next Header	8	Type of the next extension header or upper-layer protocol
Hop Limit	8	Decremented on forwarding; limits packet lifetime in hops
Source address	128	Sending interface address
Destination address	128	Destination address

Yaad rakho: IPv6: 40 fixed; payload field excludes those 40.