IPv6 datagrams use a structure that includes a regular header and, optionally, one or more extension headers. This regular header is like the header of IPv4 datagrams, though it has a different format, as you will see shortly. The standards don't give this header a name; it is just "the IPv6 header." To differentiate it from IPv6 extension headers, I call it the main header.
The IPv6 main header is required for every datagram. It contains addressing and control information that are used to manage the processing and routing of the datagram. The main header format of IPv6 datagrams is described in Table 26-2 and illustrated in Figure 26-2.
Table 26-2. IPv6 Main Header Format