Q. Subnet layer of the TCP-IP model?

These two layers of the OSI correspond straight to the subnet layer of the TCP/IP model. Majority of the time the lower layers under the Interface or Network layer of the TCP/IP model are seldom or rarely discussed. The TCP/IP model does nothing however to highlight the fact the host has to connect to the network using some protocol so it can send IP packets over it. For the reason that the protocol used isn't defined it will vary from host to host and network to network

• Subsequent to much deliberation by organizations it was decided that the Network Interface Layer in the TCP/IP model corresponds to a combination of the OSI Data Link Layer and network specific functions of the OSI network layer (example IEEE 203.3).
• Since these two layers deal with functions that are therefore inherently specific to each individual networking technology the layering principle of grouping them together related functions is largely irrelevant.