User Manual

Network architecture
BACnet architecture (MLN & ALN)
14
192 | 353 CM110664en_07
RFC1918 defines three specific address areas for private networks. IP
addresses within these ranges are not routed:
10.0.0.0 - 10.255.255.255 Subnet mask: 255.0.0.0
172.16.0.0 - 172.31.255.255 Subnet mask: 255.240.0.0
192.168.0.0 - 192.168.255.255 Subnet mask: 255.255.0.0
For IPv6, IP addresses and private address ranges are defined differently. See
Ethernet, TCP/IP, MS/TP and BACnet basics (CM110666).
IP address : Host address of the network subscriber.
Subnet mask : Subnet mask of the IP segment in which the device is located.
This value must be aligned with the other IP devices.
The subnet mask is required for the identification of broadcast messages and
for communication across IP segments. The subnet mask and target IP address
enable the transmitting IP device to decide whether the data packet can be
delivered directly to the target device or if it must be forwarded via the default
gateway.
For IPv6, the subnet mask corresponds to the network prefix. See
Ethernet,
TCP/IP, MS/TP and BACnet basics
(CM110666).
Default gateway : IP address of the IP router. This value is relevant for
communication across IP segments.
For BACnet/IP to use UDP, a UDP port number must be defined. Only devices
with the same UDP port number can communicate with each other.
Port numbers are divided into the following classes by the IANA (Internet
Assigned Numbers Authority):
● Well Known Port Numbers: Fixed port numbers assigned by IANA (0…
1023)
● Registered Port Numbers: Numbers registered with IANA (1024…48151)
● Dynamic and/or Private Ports Dynamically assigned or privately used port
numbers (49152…65535)
For BACnet, port number
47808 (0xBAC0) is registered with IANA.
If there are several BACnet internetworks on an IP network, they can be
separated by different port numbers. Using several internetworks can be
helpful in very large projects, for migration, and to encapsulate sections of a
plant with different reliability criteria. Since Desigo CC communicates
simultaneously with multiple internetworks, the operation is not restricted.
However, only one port number is registered for BACnet with the IANA. If
additional UDP port numbers are required, we recommend the use of port
numbers 47809 to 47823 (0xBAC1...0xBACF). This does not comply with IANA
regulations. This range is reserved for future applications and should not be
used. There is only a very small chance that these ports might be used
elsewhere. To avoid clashes, do not use any port numbers from the range of
dynamic or private ports. See www.iana.org/assignments/port-numbers.
The BBMD is required as soon as IP routers are used in a BACnet network. IP
routers limit broadcast messages to the local IP segment, that is, they do not
allow any broadcast messages to pass through. In order to distribute BACnet
broadcast messages across IP segments irrespective of this limitation, a BBMD
is required in the relevant IP segments. If a BBMD receives a broadcast
message, e.g., within the local IP segment, it transmits this as a unicast
message to all other BBMDs. The BBMDs then transmit the received message
to their own local IP segments. BACnet refers to this as two-hop distribution:
1. Hop: BBMD sends a unicast message to all other BBMDs.
2. Hop: They then distribute the message to all BACnet devices in the local IP
segment.
One-hop distribution can be implemented with Direct Broadcasts. In this case
the BBMD sends a direct broadcast to all remote IP segments. This broadcast
is received by all IP bus subscribers in the relevant segment. Not all IP routers
support Direct Broadcasts.
UDP port number
BACnet Broadcast
Management Device
(BBMD)