We are often faced with third-party devices in the field whose IP cannot be modified and whose IP is not in the same subnet as the BACRouter or other BIP devices.
The solution already in place is to add an IP router, which will result in increased cost and complexity.
Usually we only need the BACRouter to communicate with this third party device using Modbus TCP or BACnet IP protocol, so the easiest way is to add an IP to the BACRouter dedicated to communicate with this device

For example, local IP subnet is 192.168.100.0/24,  BACRouter’s IP is 192.168.100.1;  Third-party device has a IP of 172.16.1.20, its netmask is 255.255.255.0

We will add IP of 172.16.1.1 to BACRouter
Original /etc/rc.local:

root@OpenWrt:~# cat /etc/rc.local
# Put your custom commands here that should be executed once
# the system init finished. By default this file does nothing.

cd /root
./webui&
exit 0

Modify /etc/rc.local: (vi is available too)

root@OpenWrt:~# cat>/etc/rc.local
# Put your custom commands here that should be executed once
# the system init finished. By default this file does nothing.

ifconfig br-lan:1 172.16.1.1 netmask 255.255.255.0
cd /root
./webui&
exit 0
CTRL+D

The ifconfig command will create a br-lan:1 interface with IP 172.16.1.1; if we connect to the third-party device by BIP protocol, we have to add new interface to BACRouter configuration.

Original /root/resource.conf:

~#cat /root/resource.conf
{ “eth0”: { “type”: “ETH”, “ifname”: “br-lan” }, “RS485-1”: { “type”: “USB”, “ifname”: “0:0” }, “RS485-2”: { “type”: “USB”, “ifname”: “0:1” } }

Modify /root/resource.conf: (vi is available too)

~#cat >/root/resource.conf
{ “eth0”: { “type”: “ETH”, “ifname”: “br-lan” }, “eth1”:{“type”:”ETH”, “ifname”:”br-lan:1″}, “RS485-1”: { “type”: “USB”, “ifname”: “0:0” }, “RS485-2”: { “type”: “USB”, “ifname”: “0:1” } }
CTRL+D

now restart BACRouter!