Editing IPv6 (section)

==IPv6 packets==
{{Main|IPv6 packet}}
[[File:Ipv6 header.svg|thumb|IPv6 packet header]]

An IPv6 packet has two parts: a [[Header (computing)|header]] and [[Payload (computing)|payload]].

The header consists of a fixed portion with minimal functionality required for all packets and may be followed by optional extensions to implement special features.

The fixed header occupies the first 40&nbsp;[[octet (computing)|octets]] (320 bits) of the IPv6 packet. It contains the source and destination addresses, traffic class, hop count, and the type of the optional extension or payload which follows the header. This ''Next Header'' field tells the receiver how to interpret the data which follows the header. If the packet contains options, this field contains the option type of the next option. The "Next Header" field of the last option points to the upper-layer protocol that is carried in the packet's [[payload (computing)|payload]].

The current use of the IPv6 Traffic Class field divides this between a 6 bit [[Differentiated Services Code Point]]{{Ref RFC|2474}} and a 2-bit [[Explicit Congestion Notification]] field.{{Ref RFC|3168}}

Extension headers carry options that are used for special treatment of a packet in the network, e.g., for routing, fragmentation, and for security using the [[IPsec]] framework.

Without special options, a payload must be less than {{gaps|64|kB}}. With a Jumbo Payload option (in a ''Hop-By-Hop Options'' extension header), the payload must be less than 4&nbsp;GB.

Unlike with IPv4, routers never fragment a packet. Hosts are expected to use [[Path MTU Discovery]] to make their packets small enough to reach the destination without needing to be fragmented. See [[IPv6 packet#Fragmentation|IPv6 packet fragmentation]].