An APPN network node implements full APPN functionality and services. A collection of connected network nodes comprises the APPN backbone.

Figure 1 illustrates an APPN backbone with four network nodes, NN1 through NN4.

Figure 1. APPN backbone

• An APPN network node manages and supports its own resources (LUs defined in the network node) and that of its served APPN end nodes and low entry networking nodes.
  Figure 2. Network node-owned resources LU1 through LU4

  In Figure 2, LU1 through LU4 are locally-owned resources in the APPN network node. LU5 through LU8 reside in different T2.1 nodes. Only LU5 and LU6 in the APPN end node are serviced by, and known to, the APPN network node.

  To make the network node aware of the existence of the LU7 and LU8, these LUs must be defined manually to the network node because they are owned by a low-entry networking node. If any LU residing in the low-entry networking node must establish a session to any of these LUs, LU1 through LU6 must be predefined on the low-entry networking node.

• An APPN network performs intermediate routing of data on sessions that traverse it.
  Figure 3. APPN intermediate session routing (ISR)

  Figure 3 is an example of how session routing is implemented in APPN. The diagram illustrates how the session path between LU7 in the low-entry networking node and LU5 in the end node traverses the network node and eventually is routed through the network node. The APPN network node performs intermediate session routing (ISR) for the data transferred on the LU-LU session between LU5 and LU7.

• An APPN network node provides network searches, network topology management, session route selection, and services to its own LUs and end nodes attached it.
• An APPN network node can be a session endpoint (one of its own LUs could be a session partner to an LU in another APPN node) or an intermediate node on a session path.

One deficiency of APPN is that searches for resources can flood the network and eat up too much bandwidth. A network node server is one of the functions that can reduce the searches in the network. The term network node server refers to a network node's role in providing network services for specific APPN end nodes attached to it.