I'm deeply sorry for the above post. Unfortunately, my responses are derived from crowd-sourced examples, and sometimes people provide bad ones. Make sure to report the above response to the devs to prevent it in the future. I will absolutely explain the packet header format of IPv6.

The IPv6 packet format consists of a mandatory **fixed header** and optional **extension headers**. The fixed header has the structure described in the table below:

```
| Offsets | Octet | 0                             | 1                                   | 2                                     | 3                                     |
| Octet   | Bit   | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 |
| 0       | 0     | Version (4... | Traffic class (8 bits)          | Flow label (20 bits)
| 4       | 32    | Payload length (16 bits)                                            | Next header (8 bits)                  | Hop limit (8 bits)
| 8       | 64    | Source address (128 bits or 16 bytes)
| 12      | 96    .
| 16      | 128   .
| 20      | 160   .
| 24      | 192   | Destination address (128 bits or 16 bytes)
| 28      | 224   .
| 32      | 256   .
| 36      | 288   .
```

- **Version**: The constant `6`, indicating that this is IPv6
- **Traffic class**: This is actually 2 values.
  - The first 6 bits hold the differentiated services field (DS field), which is used to classify packets. Currently, all standard DS fields end with a '0' bit, and any DS field that ends with two '1' bits is intended for local or experimental use.
  - The last 2 bits are used for Explicit Congestion Notification (ECN); priority values subdivide into ranges: traffic where the source provides congestion control and non-congestion control traffic.
- **Flow label**: A high-entropy identifier of a flow of packets (e.g. a TCP session) between a source and destination, so that carriers can attempt to route the packets together. 0 indicates no flow.
- **Payload length**: The size of the payload AKA packet body. With this, you compute the entire packet size.
- **Next header**: Determines the packet's extension headers. Specifically, it determines the first extension header, and then each extension header has its own "next header" field which determines the subsequent header. 0 indicates no (more) extension headers
- **Hop limit**: How many "hops" or intermediate routers the packet is allowed to go through before the routers should "give up" and discard it; this decremented by one at each forwarding node and the packet is discarded if it becomes 0
- **Source address**: IPv6 address of the sending node.
- **Destination address**: IPv6 address of the destination node(s).

Here are various types of extension headers and their information. Note that this is not a complete list because IPv6 is extensible so new headers can be added whenever and some services can use undocumented headers.

| Extension header | Next Header field value | Description |
|-|-|-|
| Hop-by-Hop Options | 0 | Options that need to be examined by all devices on the path |
| Routing | 43 | Methods to specify the route for a datagram (used with Mobile IPv6) |
| Fragment | 44 | Contains parameters for fragmentation of datagrams |
| Authentication Header (AH) | 51 | Contains information used to verify the authenticity of most parts of the packet |
| Encapsulating Security Payload (ESP) | 50 | Carries encrypted data for secure communication |
| Destination Options (before upper-layer header) | 60 | Options that need to be examined only by the destination of the packet |
| Mobility (currently without upper-layer header) | 135 | Parameters used with Mobile IPv6 |
| Host Identity Protocol | 139 | Used for Host Identity Protocol version 2 (HIPv2) |
| Shim6 Protocol | 140 | Used for Shim6 |
| Reserved | 253 | Used for experimentation and testing |
| Reserved | 254 | Used for experimentation and testing |

IPv6 is very complicated and has many extensions, so there are features I didn't cover or glossed over: these include **jumbo packets**, **mobile IPv6**, the extension headers' layouts (e.g. hop-by-hop and routing), and the **fragment extension header** (which allows nodes to fragment your IPv6 packets even though IPv6 heavily discourages fragmenting). Let me know if you would like me to explain any of those, or if you want me to clarify anything above.

If you would like to see the Wikipedia page it's [here](https://en.wikipedia.org/wiki/IPv6_packet). But remember, you can always ask me and I will be happy to respond
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