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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Version 6 (IPv6) / IP Next Generation (IPng)                      9  IPv6 Datagram Encapsulation and Formatting

IPv6 Datagram Overview and General Structure 1 2 3 IPv6 Datagram Extension Headers

IPv6 Next Header Field

The Next Header field is one of the most important additions to the IPv6 datagram format. When an IPv6 datagram uses extension headers, this field contains an identifier for the first extension header, which in turn uses its own Next Header to point to the next header, and so on. The last extension header then references the encapsulated higher-layer protocol—since the higher-layer protocol’s header appears at the start of the IPv6 Data field, it is like the “next header” to the device receiving the datagram. For some folks this is a bit tough to see conceptually; much more detail on how the field works (including a useful illustration) can be found in the next topic, describing extension headers. Some of the most common values for Next Header in IPv6 are shown in Table 69.

The total length of the main IPv6 header format is 40 bytes. This is double the size of the IPv4 header without options, largely because of the extra 24 bytes needed for the monstrous IPv6 addresses. There are only 8 bytes of non-address header fields in the IPv6 main header, compared to 12 in the IPv4 header.

IPv6 Datagram Overview and General Structure 1 2 3 IPv6 Datagram Extension Headers