BACnet/IP is a different beast entirely. BACnet/IP devices view the IP internet as if it were a local area network. A device's IP address (a 4-octet number like 188.8.131.52) serves the same purpose as a device's MAC or physical LAN address in other BACnet networks (and in the BACnet network layer protocol control information). In order to control all of the functions that the SSPC wanted BACnet/IP to provide, the committee came up with the idea of a "BACnet Virtual Link Layer" or BVLL. The BVLL provides a set of messages that are used to deal, among other things, with specific idiosyncrasies of IP networks, such as the way broadcasts are handled. More on this in a moment.
The committee soon recognized another great, and initially unsuspected, benefit of the BVLL concept: the BVLL control information can be easily extended to encompass virtually any kind of new network technology or other "microprotocol" that might come along. (A microprotocol is a set of rules that provide a "value-added" function like data encryption or data compression on packets that are otherwise ready for transport.) This means that with a minimum of fuss a specification could be developed to run BACnet directly over Asynchronous Transfer Mode (ATM) networks, Synchronous Optical Networks (SONET), Frame Relay networks, Integrated Services Digital Networks (ISDN), etc., and to provide for enhanced security and efficiency measures, all without touching BACnet's existing application and network layer protocols.