The application layer handles information transfer between two network applications, including remote file services, message handling for e-mail, and remote database access. Many of the services under this layer are called application programming interfaces (APIs). APIs are programming libraries used to write applications.
The OSI model layers are usually numbered in the order listed in figure C-1. So if you see a reference to Layer 3 of the OSI model, you'll know it's the network layer being described.
The OSI reference model is useful because every networking protocol stack is based in some way on the OSI structure. None of the protocol stacks you encounter will fit the model exactly, but there's always enough correspondence to help you understand the relationship among the parts—and certainly enough to get you started if you want to delve deeper into the technical core of networking.
Microsoft's TCP/IP protocols follow a four-layer conceptual model, also called the PC model, that combines some of the layers, as shown in Table C-1. The layers are simplified, but the same elements are present in the Microsoft TCP/IP stack as in the traditional OSI model.
Table C-1. Comparing conceptual models
|Microsoft Model||OSI Layers|
|Network interface layer||Physical and data link layers|
|Internet layer||Network layer|
|Transport layer||Transport layer|
|Application layer||Session, presentation, and application layers|