The TCP/IP Guide - IPv6 Datagram Extension Headers

Table 70 lists the different extension headers, showing its Next Header value, length and defining RFC, and providing a brief description of how each is used.

**Table 70: IPv6 Extension Headers**
Next Header Value (decimal)	Extension Header Name	Length (bytes)	Description	Defining RFC
0	*Hop-By-Hop Options*	Variable	Defines an arbitrary set of options that are intended to be examined by all devices on the path from the source to destination device(s). This is one of two extension headers used to define variable-format options.	2460
43	*Routing*	Variable	Defines a method for allowing a source device to specify the route for a datagram. This header type actually allows the definition of multiple routing types. The IPv6 standard defines the Type 0 Routing extension header, which is equivalent to the “loose” source routing option in IPv4 and used in a similar way. See below for the format of this extension header.	2460
44	*Fragment*	8	When a datagram contains only a fragment of the original message, this extension header is included. It contains the Fragment Offset, Identification and More Fragment fields that were removed from the main header. See below for the format of this extension header, and the topic on fragmentation and reassembly for details on how the fields are used.	2460
50	*Encapsulating Security Payload (ESP)*	Variable	Carries encrypted data for secure communications. This header is described in detail in the section on IPSec.	2406
51	*Authentication Header (AH)*	Variable	Contains information used to verify the authenticity of encrypted data. This header is described in detail in the section on IPSec.	2402
60	*Destination Options*	Variable	Defines an arbitrary set of options that are intended to be examined only by the destination(s) of the datagram. This is one of two extension headers used to define variable-format options.	2460

Note that the Next Header value of the IPv6 main header is 41; that of an IPv4 header is 4 (its protocol number). Note that there is also a “dummy” extension header called No Next Header that has a value of 59. This is a placeholder that when found in the Next Header field indicates that there is nothing after that extension header.

As I mentioned in the table, the formats for several of the headers are provided in other topics. Two of them I will describe here, however.