In addition to devices talking directly to each other, there needs to be a mechanism to broadcast a message so that all B/IP devices receive it. We will use the existing standard for broadcasting UDP packets.
A UDP broadcast packet is 'sent' to an IP address with the subnet of the broadcasting device and all 1's in the host portion. For example, if a device has an address 126.96.36.199 and a subnet mask 255.255.255.0, it can use the broadcast address 188.8.131.52. Although most networks also allow the use of the IP broadcast address 255.255.255.255, we have ruled this out for BACnet/IP because of some IP protocol stack limitations that certain implementors have encountered.
The BVLCI indicates that this is an Original-Broadcast-NPDU.
The NPCI shows that this is not "data expecting a reply," i.e., a broadcast.