The OSI model revisited
Network Layer performs addressing and routing (it controls levels 1,2 and 3 = sub-network); software accepts messages from layer 4 and ensures that the packages are directed to their proper destination. After routing, the packet is passed down to the Data Link Layer
Transport Layer (host-to-host layer) establishes, maintains, and terminates
logical connections; generates the address of the end user; breaks large data transmission into smaller packets if needed; ensures packets have been received; eliminates duplicate packets;
deals with end-to-end issues (entering and departing the network); multiplexing; inverse multiplexing; concerned of messages (lover levels concerned of packets)