IPv6 Packet header details.

IPv6 Header.

In IPv6 packet, the header length has a fixed length of 40 bytes, or 320 bits (1 bytes = 8 bits). Today we will take a closer look on the fields in the IPv6 packet header.

Version
The first 4-bit field contains the version of the protocol or in another words, it is to indicate the version of the Internet protocol. In the case of IPv6 packet, the version number is 6 or 0110 in bit sequence.

Traffic Class
This is a 8-bit field and it replaces the Type of Service (ToS) in IPv4 header. This field defines the class-of-service (CoS) priority of the packet.
This field can be used by sending nodes and forwarding routers to identify between different priorities of IPv6 packets.

Flow Label
This is a 20-bit field and it is to identify all packets belonging to a specific flow. A sending nodes can use this field to label those packets that requires special handling by the IPv6 routers. Routers can identify these packets and handle them in a similar fashion. In short, it is for QoS management.

Payload Length
This is a 16-bit field and it is to specify the length of the IPv6 payload. This field replace the Total Length field in IPv4. The calculation of the Total Length field in IPv4 is not the same with the Payload Length in IPv6 header. The Total Length field in IPv4 includes the length of the IPv4 header, wherease the Payload Length in IPv6 does not includes the length of IPv6 header. Please be aware that the Extension Headers are not part of the IPv6 header, therefore Extension Headers are included in the calculation of Payload Length in IPv6.

Next Header
This is a 8-bit header and it is to indicate the type of next header that immediately follows the IPv6 header. It replaced the Protocol field in IPv4 header. As for the contents of this field, it contains the same protocol numbers as in IPv4.

Hop Limit
This is a 8-bit field and is analogous to the TTL field in IPv4. It is to indicates the maximum number of hops allowed. It will be decremented by 1 by each nodes that forwards the packets. Node will not forwards the packet and will discards it when the value of Hop Limit is decremented to 0.

Source Address
This is a 128-bit field and it contains the IPv6 address of the initial sender of the packet.

Destination Address
This is a 128-bit field and it contains the IPv6 address of the intended recipient of the packet. It is a slightly different between the IPv4 and IPv6 Destination field where in IPv4, the Destination field contain the IPv4 address of the final destination, but in IPv6, the Destination field might not necessary contains the final destination address if a Routing header is present.