The function of the application layer is to interface between an external program application and the network stack. As such it has to build the first part of the packet that describes the protocol being used in the network.
There are a series of API (application Program Interfaces) which allows a software developer to interface with the OSI layers without knowing anything about how to network.
OSI stands for Open system interconnection model. Application layer is the seventh layer in OSI model.
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application layer
The top layer of the OSI model; Application
Networking layer of the OSI model
Layer 7: The Application Layer
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Application
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Application Layer
Application Layer