See Also
 Important: |
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An IPv6 packet, also known as an IPv6 datagram, consists of an IPv6 header and an IPv6 payload, as shown in the following illustration.
The IPv6 header consists of two parts, the IPv6 base header, and optional extension headers. Functionally, the optional extension headers and upper-layer protocols, for example TCP, are considered part of the IPv6 payload. The following table shows the fields in the IPv6 base header.
| IPv6 header field | Length | Function |
|---|---|---|
|
Version |
4 bits |
Identifies the version of the protocol. For IPv6, the version is 6. |
|
Class |
8 bits |
Intended for originating nodes and forwarding routers to identify and distinguish between different classes or priorities of IPv6 packets. |
|
Flow label |
20 bits |
Defines how traffic is handled and identified. A flow is a sequence of packets sent either to a unicast or a multicast destination. This field identifies packets that require special handling by the IPv6 node. The following list shows the ways the field is handled if a host or router does not support flow label field functions:
|
|
Payload length |
16 bits |
Identifies the length, in octets, of the payload This field is a 16-bit unsigned integer. The payload includes the optional extension headers, as well as the upper-layer protocols, for example, TCP. |
|
Next Header |
8 bits |
Identifies the header immediately following the IPv6 header. The following list shows examples of the next header:
|
|
Hop Limit |
8 bits |
Identifies the number of network segments, also known as links or subnets, on which the packet is allowed to travel before being discarded by a router. The Hop Limit is set by the sending host and is used to prevent packets from endlessly circulating on an IPv6 internetwork. When forwarding an IPv6 packet, IPv6 routers must decrease the Hop Limit by 1, and must discard the IPv6 packet when the Hop Limit is 0. |
|
Source Address |
128 bits |
Identifies the IPv6 address of the original source of the IPv6 packet. |
|
Destination Address |
128 bits |
Identifies the IPv6 address of the intermediate or final destination of the IPv6 packet. |
IPv4 headers and IPv6 headers are not interoperable. A host or router must use an implementation of both IPv4 and IPv6 in order to recognize and process both header formats.