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This section covers each field of the IPv6 header. The IPv6 header is simpler than the IPv4 header. Some IPv4 fields have been eliminated or changed to optional fields. The fragment offset fields and flags in IPv4 have been eliminated from the header. IPv6 adds a flow label field for quality-of-service (QoS) mechanisms to use.

The use of 128 bits for source and destination addresses provides a significant improvement over IPv4. With 128 bits, there are 3.4 * 10³⁸ or 34 billion billion billion billion IPv6 addresses, compared to only 4.3 billion IPv4 addresses.

IPv6 improves over IPv4 by using a fixed-length header. The IPv6 header appears in

The following is a description of each field in the IP header:

• Version— This field is 4 bits long. It indicates the format, based on the version number, of the IP header. These bits are set to 0110 for IPv6 packets.
• Traffic class— This field is 8 bits in length. It describes the class or priority of the IPv6 packet and provides functionality similar to the IPv4 type-of-service field.
• Flow label— This field is 20 bits in length. It indicates a specific sequence of packets between a source and destination that requires special handling, such as real-time data (voice and video).
• Payload length— This field is 16 bits in length. It indicates the payload's size in bytes. Its length includes any extension headers.
• Next header— This field is 8 bits in length. It indicates the type of header that follows this IPv6 header. In other words, it identifies the upper-layer protocol. It uses values defined by the Internet Assigned Numbers Authority (IANA).
• Hop limit— This field is 8 bits in length. It is decremented by 1 by each router that forwards the packets. If this field is 0, the packet is discarded.
• Source address— This field is 128 bits in length. It indicates the sender's IPv6 address.
• Destination address— This field is 128 bits in length. It indicates the destination host's IPv6 address.

Notice that although the IPv6 address is four times the length of an IPv4 address, the IPv6 header is only twice the length (40 bytes). Optional network layer information is not included in the IPv6 header; instead, it is included in separate extended headers.

Two important extended headers are the Authentication Header (AH) and the Encapsulating Security Payload (ESP) header. These headers are covered later in the chapter.