The data payload for an IP datagram carries the actual data or message that is being sent over the network. The IP datagram is conceptually divided into two pieces: the header and the payload. The header contains addressing and control fields, while the payload carries the actual data to be sent over the network. The IP datagram header contains information such as the source and destination IP addresses, the protocol being used (e.g. TCP, UDP), and other control information. The payload can vary in size depending on the type of data being sent, for example, sending a small text message would have a smaller payload size compared to sending a large video file.