Q: In terms of licensing, what qualifies as a “device?”
A: A device can be any physical or virtual instance that has a MAC address that identifies what it is (i.e. router, switch, server, access point). Multiple instances of the same device can be combined in a Probe Group on the same map. Instances of the same device on different maps can share polling (and be counted as a single device) if they match in probe type and configuration as outlined here.
Devices are added to the map using an IP address. If a device with multiple IP addresses can be monitored using one IP address, it doesn’t matter how many others are on the device. It’s still a single device for licensing purposes.
Switch stacks actually fit this category; if you have a probe that will monitor the vendor’s stack MIB, you can get information about all the switches in the stack through a single IP address. That would require a custom probe, or maybe there is a user-contributed one available.
Another case is where multiple virtual devices (virtual switches, virtual routers) are configured in a single chassis. Each of them acts independently, has its own management IP address, and would be a separate device for licensing purposes.
In some scenarios when using autodiscover, multiple copies of a device may be added to the map at different IP addresses. Identifying and removing duplicates is easily done, either using DNS resolution or by setting the device labels to show sysName as the label, then sorting the devices in list view.
So the question is; can the whole device can be monitored using a single IP.