What is a packet?

A packet is chunk of information that is exchanged between computers on a network. A packet can contain data, control codes, messages, or commands.

Packets originate at the Application Layer where the information is organized into Protocol Data Units (PDUs). As the PDU moves down through the protocol stack, each layer attaches its own information to the PDU until it arrives at the Data Link Layer where it is encoded into data frames for transmission. Once it arrives at its destination, the PDU moves up the protocol stack, each layer "unwrapping" the information attached by its peer layer on the sending side. This is how peer protocol layers communicate.

Although the terms are often used interchangeably, a packet is not necessarily the same as a data frame. A packet can be segmented into several frames. Data is transmitted in packets (PDUs) through the protocol stack and then as data frames over the physical connection.