IPv6 Datagram Overview and General Structure

The method by which IPv6 encapsulates data received from higher-layer protocols for transmission across the internetwork is basically the same as the one used by IPv4. The data received from the transport or higher layers is made the payload of an IPv6 datagram, which has one or more headers that control the delivery of the message. These headers provide information to routers in order to enable them to move the datagram across the network. They also provide information to hosts so they can tell which datagrams they are intended to receive.

While the basic use of datagrams hasn't changed since IPv4, many modifications were made to their structure and format when IPv6 was created. This was done partly out of necessity: IPv6 addresses are different from IPv4 addresses, and IP addresses go in the datagram header. The increase in the size of IP addresses from 32 bits to 128 bits adds a whopping extra 192 bits, or 24 bytes, of information to the header. This led to an effort to remove fields that weren't strictly necessary in order to compensate for the necessary increase in size. However, changes were also made to IPv6 datagrams to add features to them and to make them better suit the needs of modern internetworking.