Packets consist of control information for addressing and routing, and a payload consisting of user data. The control information in IPv6 packets is subdivided into a mandatory fixed header and optional extension headers. The payload of an IPv6 packet is typically a datagram or segment of the higher-level Transport Layer protocol, but may be data for an Internet Layer (e.g., ICMPv6) or Link Layer (e.g., OSPF) instead.
IPv6 packets are typically transmitted over a Link Layer protocol, such as Ethernet which encapsulates each packet in a frame, but this may also be a higher layer tunneling protocol, such as IPv4 when using 6to4 or Teredo transition technologies.
Routers do not fragment IPv6 packets, as they do for IPv4. Hosts are "strongly recommended" to implement Path MTU Discovery to take advantage of MTUs greater than the smallest MTU of 1280 octets. A node may use the IPv6 Fragment header to fragment the packet at the source and have it reassembled at the destination(s).
Since July 2017, the Internet Assigned Numbers Authority (IANA) is responsible for registering all IPv6 parameters that are used in the various IPv6 headers.