APPN networks provide open connectivity, and require minimal configuration by each system in the network. When a system has a connection into an APPN network, it can establish sessions with other systems that are connected within that APPN network.
APPN reduces the physical, configuration barriers to communications. However, you might want to build some logical barriers between systems in the network for security reasons. This ability to control which systems can connect to yours is often called firewall support. Network administrators might use a variety of node types to specify which connections between APPC locations are allowed. For example, you might want to allow SYSTEMB to communicate with SYSTEMA and SYSTEMD, but not with SYSTEMC. For an explanation of this, an example, and more information, see the following topics.