Glossary · Networking

Packet (Network Packet)

A packet is a small unit of data with a header and payload that travels independently across a network, such as an IP packet.

Updated October 6, 2026

A packet is a formatted unit of data sent across a network. It has a header, containing control information such as source and destination addresses, and a payload, the portion of the actual data being carried. Large messages are split into many packets and reassembled at the destination.

Why it matters

Packet switching, rather than reserving a dedicated line, is how the internet shares links efficiently among many users. Each packet can take a different path and arrive out of order, so protocols like TCP number the pieces and reorder them. Packets also set limits: an Ethernet link typically carries up to 1500 bytes of IP packet (MTU), and each router drops packets when queues overflow, which is packet loss.

Strictly, a layer-2 unit is a frame, a layer-4 TCP unit is a segment and UDP's is a datagram, though "packet" is used loosely for all.

Example

A 3 KB web response is split into about three TCP segments, each wrapped in an IP packet (20-byte IPv4 header), then in an Ethernet frame. Capturing with tcpdump shows lines like IP 203.0.113.10.443 > 192.168.1.23.51514: Flags [P.], length 1448.

Common mistakes

  • Assuming all packets of one transfer follow the same route.
  • Treating packets and frames as identical when debugging at different layers.
  • Forgetting header overhead when calculating real throughput.

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