# Q4A · IPv6 packet format

Subject: Computer Networks

Exam: M2

Reader: https://deep-learning-study-room.vercel.app/cn/#m2/papers/paper-2026-reexam/q4a

Canonical: https://deep-learning-study-room.vercel.app/read/cn/m2/papers/paper-2026-reexam--q4a/

Actual TEE paper · current-M2 matching parts; original marks

### Actual PYQ. Construct the IPv6 packet format and explain the purpose of each field. [10 marks]

#### Answer

Swipe across the diagram to read its labels ↔

![IPv6 base header · each horizontal row is 32 bits wide](https://deep-learning-study-room.vercel.app/cn/fig/ipv6-header.aef1bfbac7.svg)

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)

Eight base-header fields occupy 320 bits = 40 bytes.

| 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 |

The first two 32-bit rows contain 64 bits and the two addresses contain 256 bits, giving a 320-bit base header. Payload Length excludes these 40 bytes. Next Header can identify an extension header or upper-layer protocol. IPv6 has no base-header checksum; optional information is carried through extension headers.

#### Practical example

A packet with 24 extension-header bytes and 1,200 upper-layer bytes has Payload Length=1,224 and total length=1,264 bytes. This illustrates why the two length values differ.