What does APPN support in networking?

APPN, or Advanced Peer-to-Peer Networking, primarily facilitates computer connections at the network and transport layers, making it integral to network communications. APPN functions to enable devices on a network to communicate effectively, especially in IBM's networking environments where it plays a crucial role in establishing peer-to-peer connections.

This means that it supports the establishment of communication pathways that allow for the transfer of data and other services across the network. By focusing on the transport and network layers, APPN ensures reliable delivery of messages and efficient use of network resources, which are pivotal for large-scale networking environments.

Other options such as mobile connections, data encryption, and file transfer optimization, while important in networking contexts, do not accurately describe the primary function of APPN. APPN is not specifically designed for mobile and stationary device connections, nor is it primarily concerned with encryption or optimizing file transfers across various networks. Instead, its main focus is on enabling structured and robust connections between peers in a data network.