Network Number

A BACnet network, formally, a "BACnet Internetwork" consists of multiple "BACnet Networks".

Each BACnet Network is given a 16-bit Network Number (NN) to allow devices on the network to be addressed via their BACnetAddress. (A BACnetAddress is NN:BACnetMAC)

Refer to the specification "6.2.2.1 DNET, SNET, and Vendor ID Encoding" where it clarifies that the NN for the DNET cannot be 0 and the NN for the SNET cannot be 0 or 65535.

The DNET can be 65535 since this indicates a Global Broadcast.