The four layers in the DoD model, from bottom to top, are:

1. The Network Access Layer is responsible for delivering data over the particular hardware media in use. Different protocols are selected from this layer, depending on the type of physical network.

2. The Internet Layer is responsible for delivering data across a series of different physical networks that interconnect a source and destination machine. Routing protocols are most closely associated with this layer, as is the IP Protocol, the Internet’s fundamental protocol.

3. The Host-to-Host Layer handles connection rendezvous, flow control, retransmission of lost data, and other generic data flow management. The mutually exclusive TCP and UDP protocols are this layer’s most important members.

4. The Process Layer contains protocols that implement user-level functions, such as mail delivery, file transfer and remote login.