IPv6 Essentials

The Fields in the IPv6 Header

By becoming familiar with the fields of the IPv6 header, you will better understand how IPv6 works.

Tip

For a detailed description of all the fields in an IPv4 header, refer to Novell’s Guide to Troubleshooting TCP/IP (John Wiley & Sons) by Silvia Hagen and Stephanie Lewis.

Figure 2-1 provides an overview of the IPv6 header. The fields are discussed in detail in the following paragraphs.

Figure 2-1. Fields in the IPv6 header

Figure 2-1 shows that even though the header has a total size of 40 bytes, which is twice as long as a default IPv4 header, it has actually been streamlined because most of the header is taken by the two 16-byte IPv6 addresses. That leaves only 8 bytes for other header information.

Version (4 Bits)

This is a 4-bit field and contains the version of the protocol. In the case of IPv6, the number is 6. The version number 5 could not be used because it had already been assigned an experimental stream protocol (ST2, RFC 1819).

Traffic Class (1 Byte)

This field replaces the Type of Service field in IPv4. This field facilitates the handling of real-time data and any other data that requires special handling. This field can be used by sending nodes and forwarding routers to identify and distinguish between different classes or priorities of IPv6 packets.

RFC 2474, “Definition of the Differentiated Services Field (DS Field) in the IPv4 ...