User Manual

Network architecture
BACnet architecture (MLN & ALN)
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CM110664en_07 195 | 351
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.
UDP port number
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.
BACnet Broadcast Management Device (BBMD)
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.
IPv6 (BVLLv6) only supports two-hop BBMD. Broadcasts are implemented via IPv6 mutlicasts. See
Ethernet, TCP/IP, MS/TP and BACnet basics (CM110666).
BBMDs ensure that broadcast messages are distributed in a BACnet network. They are grouped by BACnet
network. A maximum of one BBMD is allowed in any one IP segment.
BACnet network #100 is separated by IP routers. The Internet also contains IP routers. This is why different
segments are shown before and after the Internet cloud. BBMDs are required so that BACnet broadcast
messages are available in all IP segments.
BBMD parameters
The BBMD parameters are written to the BBMD or (for Desigo) to the BACnet router during
commissioning. The following information is required for each BBMD in the BACnet network: