A packet is a small unit of data formatted for transport across a packet-switched network. Each packet has a predefined maximum size. When a message exceeds this size, it is divided into multiple packets. To enable efficient routing through the network, each packet is equipped with a header. The contents of the header vary depending on the protocol used for transporting the packets. However, the header must always include the destination address. Additional information in the header might encompass the source address, a sequence number for correctly reassembling the data at the receiving end, and a checksum to ensure data integrity. In some instances, packets also have a trailer, marking the packet’s end and possibly containing error detection and correction information.