Answer: (i) Host to host

The classic TCP/IP model has four layers: Application - Host-to-Host - Internet - Network Access. The Host-to-Host layer (later also called the transport layer) provides process-to-process, end-to-end delivery using port numbers and the protocols TCP and UDP - precisely the service offered by the OSI transport layer (layer 4).

The Internet layer (ii) corresponds to the OSI network layer (3): it handles IP addressing, routing and best-effort datagram delivery. The Application layer (iii) merges the OSI session, presentation and application layers (5, 6, 7). The Network Access layer (iv) merges the OSI data link and physical layers (1, 2).