In its simplest form, a packet is the basic unit of information in network transmission. Most networks use TCP/IP as the network protocol, or set of rules for communication between devices, and the rules of TCP/IP require information to be split into packets that contain both a segment of data to be transferred and the address where the data is to be sent.
Because TCP/IP was constructed to use packets, it is considered a packet-switching technology, the primary benefit of which is that data can be routed to a destination through any number of transmission points, making the network decentralized and less vulnerable to equipment failure. As a comparison, networks that use circuit-switching technologyÑlike the telephone networkÑmust set up a dedicated connection between two points, which has a larger resource footprint and is easier to disrupt.
Every packet that is transmitted over a packet-switching networkÑsuch as the Internet, the largest such network in existenceÑis constructed of two major pieces: the packet header and the data. Within the header are several distinct pieces of information about the packet itself. This information includes the version of the protocol being used (IPv4 or IPv6), the length of the packet, the number of packets used to send the total data in question, the source and destination addresses, a checksum (used in error correction calculations), and the Time To Live (TTL) data, which defines how many devices the packet may be transmitted along, or hops, before the message is allowed to time out. The data itself is divided into segments of length that can vary, generally in a range of 0 to 64 kB. Packets are transmitted over Ethernet networks, the most common physical type, within frames, or pre-set data blocks that have their own header and trailer information.
Since packets are the basic unit of network transmission, they fit into the standard model of networking model, known as the Open System Interconnection (OSI) model, at the network level, where network devices transmit and configure packet routing, or the paths a given packet will take to reach its destination. After being formed at the network layer, packets are encoded into bits, then passed down to the data link layer. From there, the packets are inserted into frames, and then passed to the physical layer, which is the actual medium of transmission. The process is reversed at the destination, with signal passing to the data link layer, pulling the data as bits from the frame, decoding into packets and passing the packets to the network layer for transmission.
Although the process of packet usage seems complicated, it actually makes the transmission of information far more reliable. Even if two devices are using a protocol that does not guarantee reliable transmission, such as User Datagram Protocol (UDP), a subprotocol of the TCP/IP suite, the fact that each packet includes the source and destination address allows devices to deliver most packets successfully or return failure information back to the source device, allowing for retransmission. Additionally, the header information that describes the number, type and error correction data of the packet allows every packet to provide information about the message as a whole, allowing for easier reconstruction and correction than is possible under circuit-switching transmissions. Short, sturdy and reliable, the structure of packets and packet-switching networks allow for fast and reliable data transmission, and make networks like the Internet possible.