What are the seven layers of the OSI model?

The OSI model has seven layers (bottom to top): Physical (bits, cables, signals), Data Link (frames, MAC addresses, switches), Network (packets, IP addresses, routing), Transport (segments, TCP/UDP, ports), Session (connection management), Presentation (data format, encryption), Application (HTTP, FTP, user interface). Each layer provides services to the layer above and uses services from below. The TCP/IP model simplifies this to 4 layers.