Bits, Frames, Packets and Segments: What Each Layer Calls Its Data

Each layer of the network stack has its own name for the unit of data it handles: bits at the physical layer, a frame at the data link layer, a packet at the network layer, and a segment or a datagram at the transport layer. The names matter because each one tells you which header is on the outside, and so which device or tool can read it.

In conversation people say "packet" for all of them, and most of the time nothing goes wrong. It goes wrong when the difference is the point. "The switch dropped the packet" sends someone to look at IP routes, when a switch only ever reads frames and the fault is in its table of MAC addresses. "The packet was too big" could mean a frame over the link's limit or a segment over the receiver's window, and those have different fixes.

The layer numbers and the units go together, so one can be read from the other. A "layer 2 problem" is a problem with frames: a cable, a switch, a Wi-Fi association. A "layer 3 problem" is a problem with packets: an address, a route, a firewall rule on IP addresses.

One packet travels inside many frames during its trip. Each link wraps the same packet in a new frame, so the frame on the first link and the frame on the last share nothing but the packet inside them.

So use the precise word whenever a fault is being described, because it tells the person reading which device to look at and which header to inspect.