A packet is a unit of data that is transmitted over a network. In the context of computer networks, a packet is a bundle of data that is broken down into smaller units for more efficient transmission over the network.

A packet typically has three parts:

  1. Header: Contains control information and routing information, such as the source and destination addresses, the packet's sequence number, and error detection and correction codes.
  2. Payload: Contains the actual data that is being sent or received.
  3. Trailer: Contains error checking and correction data, such as a cyclic redundancy check (CRC).

The header and trailer are also known as the packet's overhead, this information is used for routing the packet, error checking, and other control functions. The payload is the actual data that is being sent, such as a file, a message, or a video. The payload is usually the largest part of the packet.

In the networking protocols such as TCP/IP, the packet is called "datagram" or "segment" in TCP, and "datagram" in UDP, these protocols use packets to transmit data over the network. The packet is divided into smaller units and each unit is sent to the destination, when the destination receives all of the units, it reassembles the packet.

It's important to note that the size of the packet is determined by the protocol being used and the maximum transmission unit (MTU) of the network. The MTU is the largest packet size that a network can transmit. If the packet size exceeds the MTU, it will be divided into smaller packets (fragmented) before being sent.