Header

Version

Length: 4 bits.

Indicates the version of IP that is used.

Fixed value of 6 (0b0110) to indicate IPv6

Traffic Class

Length: 8 bits.

Used for QoS (Quality of Service), to indicate high-priority traffic.

For example, IP phone traffic, live video calls, etc, will have a Traffic Class value which gives them priority over other traffic.

Flow Label

Length: 20 bits.

Used to identify specific traffic 'flows' (communications between a specific source and destination).

Payload Length

Length: 16 bits.

Indicates the length of the payload (the encapsulated Layer 4 segment) in bytes.

The length of the IPv6 header itself isn't included, because it's always 40 bytes.

Next Header

Length: 8 bits.

Indicates the type of the 'next header' (header of the encapsulated segment), for example, TCP or UDP.

Same function as the IPv4 header's 'Protocol' field.

Hop Limit

Length: 8 bits.

The value in this field is decremented by 1 by each router that forwards it. If it reaches 0, the packet is discarded.

Same function as the IPv4 header's 'TTL' field.

Source / Destination

Length: 128 bits each

These fields contain the IPv6 addresses of the packet's source and the packet's intended destination.