You can write programs to run on a specific transport or transport type, or to operate on a system-chosen or user-chosen transport. Two mechanisms for network selection are the /etc/netconfig database and the environmental variable NETPATH. These mechanisms enable a fine degree of control over network selection – a user can specify a preferred transport and an application will use it if it can. If the specified transport is inappropriate, the application automatically tries other transports with the right characteristics.
/etc/netconfig lists the transports available to the host and identifies them by type. NETPATH is optional and enables you to specify a transport or selection of transports from the list in /etc/netconfig. By setting the NETPATH, you specify the order in which the application tries the available transports. If NETPATH is not set, the system defaults to all visible transports specified in /etc/netconfig, in the order that they appear in that file.
RPC divides selectable transports into the types described in the following table.