Advanced Peer-to-Peer Networking (APPN) is an extension to the Systems Network Architecture (SNA).
It includes features such as these:
APPN was meant to complement IBM's Systems Network Architecture, a legacy from the mainframe era. It was designed as a simplification, but it turned out to be utterly complex, in particular in migration situations. APPN was originally meant to be a "DECNET killer", but DEC actually died before APPN was completed. Today, of course, APPN has been completely superseded by TCP/IP (Internet).
Note that APPN has nothing to do with controversial peer-to-peer file sharing software such as KaZaa or Napster. The designation peer-to-peer in the case of APPN refers to its independence from a central point of control, similar to the way that a FireWire PC connection allows a video camera to talk directly to a disk drive on the FireWire network.
APPN evolved to include a more efficient data routing layer which was called High Performance Routing (HPR). HPR was made available across a range of enterprise corporation networking products in the late 1990s, but today is typically used only within IBM's z/OS environments as a replacement for legacy SNA networks. It seems to be still widely used within UDP tunnels, this technology is known as Enterprise Extender.