Packets

Packets are the formatted units of data that are transmitted over packet-switched networks. When a higher-layer message is sent, it is divided into smaller pieces, each packaged as a packet with its own header and payload. This division enables efficient use of network links, parallel transmission, and independent routing of individual packets along potentially different paths.

A packet typically contains a header with control and addressing information, followed by a payload containing a portion of the original data. Common header fields include source and destination addresses, the protocol identifier for the next layer, a length field, and a checksum or other error-detection mechanism. Some networks also attach trailers or use additional headers at other layers.

Different layers define different packet formats. For example, an IP packet (or datagram) is a network-layer unit that carries an IP header and an encapsulated payload. Inside the IP payload, transport-layer segments such as TCP segments or UDP datagrams may reside. At the data-link layer, frames carry the packets further, and the exact structure depends on the network technology (Ethernet, for instance).

Routers and switches forward packets toward their destinations based on header information. If a packet encounters a smaller maximum transmission unit along the path, it may be fragmented, with fragments reassembled at the destination. Packets can arrive out of order, requiring transport protocols (like TCP) to provide sequencing and reliability, while some protocols (like UDP) are connectionless and may lose packets.

Security considerations include the possibility of interception, spoofing, or tampering, and many networks employ encryption, authentication, and firewalls to mitigate risks. Packet-based networks underpin most digital communication, from the Internet to local area networks.