Many campuses are configured with 1 BACnet Network ID number for Primary IP that is unique per building whether or not they are in the same IP subnet or multiple (with BBMDs).
EcoStruxure Building Operation
Building Operation Automation Server
When reviewing the Technical Reference Guide and searching "BACnet and IP Routing", it does not clearly explain the configuration and it also does not extend to multi-building systems using BBMDs.
BACnet network numbers are assigned at BACnet routers, with a Range of 1-65,534 (where 65535 is reserved for broadcasts). Routed networks are assigned unique network numbers and can only have 1 router per network. Routed networks are usually BACnet MSTP, BACnet Ethernet, and Secondary IP networks.
Standard practice is to have primary IP networks consisting of IP routers/devices in the same IP subnet or across multiple IP subnets joined by a BBMD and/or foreign device configuration uses the default BACnet network number 1.