Q I have two Internet connections to my home office, to provide reliability through redundancy. They regularly get confused with each other, and I see messages such as ‘Another device on your network is using your IP 192.168.1.1’. Both my main modem-router and my AirPort base station run DHCP. How can I fix this?
A You cannot run two DHCP servers on the same network, as they will both try to hand out IP addresses that clash with one another. Select one of the two to provide DHCP services, and configure it to accept fixed IP addresses up to, say, 192.168.1.50, only assigning DHCP addresses from .51 to 250.
Next set your modem-routers to fixed IP addresses of 192.168.1.1 and .2. All computers and devices that use the network regularly should then be assigned fixed IP addresses starting from 192.168.1.3 up to .50, working through their preferred router at 192.168.1.1 or .2 as desired.
You can also set fallback Network pane settings to use the other router in the event of failure. That will prove much more robust, and fixed IP address assignments will ensure that neither you nor your DHCP server will ever get them wrong, or in conflict. Full details are given here and here.
Note that recent versions of OS X allow you to perform link aggregation with Macs which have two Ethernet ports: see this Q&A for details.
Updated from the original, which was first published in MacUser volume 29 issue 10, 2013.