Understanding IPv6 Packet Header Extensions
This topic defines IP version 6 (IPv6) packet header extensions.
IPv6 extension headers contain supplementary information used by network devices (such as routers, switches, and endpoint hosts) to decide how to direct or process an IPv6 packet. The length of each extension header is an integer multiple of 8 octets. This allows subsequent extension headers to use 8-octet structures.
Any header followed by an extension header contains a Next Header value that identifies the extension header type. Extension headers always follow the basic IPv6 header in order as shown in Table 7:
 | Note: The destination IP address can appear twice, once after the hop-by-hop header and another after the last extension header. |
Table 7: IPv6 Extension Headers
Header Name | Purpose |
|---|---|
Hop-by-Hop Options | Specifies delivery parameters at each hop on the path to the destination host. Note: A hop-by-hop option can appear only following the IPv6 basic header. If it is used, it should be the first extension header. It cannot appear after another extension header. |
Destination Options | Specifies packet delivery parameters for either intermediate destination devices or the final destination host. When a packet uses this header, the Next Header value of the previous header must be 60. |
Routing | Defines strict source routing and loose source routing for the packet. (With strict source routing, each intermediate destination device must be a single hop away. With loose source routing, intermediate destination devices can be one or more hops away.) When a packet uses this header, the Next Header value of the previous header must be 43. |
Fragment | Specifies how to perform IPv6 fragmentation and reassembly services. When a packet uses this header, the Next Header value of the previous header must be 44. A source node uses the fragment extension header to tell the destination node the size of the packet that was fragmented so that the destination node can reassemble the packet. |
Authentication | Provides authentication, data integrity, and anti-replay protection. When a packet uses this header, the Next Header value of the previous header must be 51. |
Encapsulating Security Payload | Provides data confidentiality, data authentication, and anti-replay protection for Encapsulated Security Payload (ESP) packets. When a packet uses this header, the Next Header value of the previous header must be 50. |
Destination IP Address | Identifies the host device, or interface on a node, to which the IPv6 packet is to be sent. Note: The destination address may appear twice, the first instance after the hop limit following the source IP address and the second instance after the final extension header. |