This sets the IP address used for actual traffic passing through whichever unit of a redundant pair is currently online, and the MIB notes it is only settable on that active unit. Because this address moves with the online or offline role rather than being tied to a specific physical unit, the pair presents one consistent traffic-facing IP address to the rest of the network regardless of which physical modem is currently active, which is the whole point of the redundancy scheme from the network's perspective. Attempting to set this object on the currently offline unit of a pair would fail, since the MIB explicitly restricts this setting to the online unit only.