| device | operates at | sees things at the | set of devices can communicate with | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| hub | layer 1 — physical layer | bit-level | network segment (vague) or collision domain | |||||||||||
| layer 2 — link layer | frame-level | broadcast domain (subnet may be incorrectly used, it only applies when a subnet is a leaf) | ||||||||||||
| router | layer 3 — network layer | packet-level | all nodes on that connected graph | |||||||||||
| stateful firewall, NAT | layer 4 — transport layer | connection-level | ||||||||||||
| layer 5 | a group of related connections |