Network Discovery finds hundreds of subnets

[tfines]

LAN has 2 networks: 192.168.15.0/24 and 192.168.2.0/24.  NetXMS is 5.1.0/Windows.

When I run a discovery (active & passive), NetXMS finds over 400 additional subnets.  None of the subnets have devices found in them; netxms only finds the subnets.  See attached image for what I mean.

I have my 'Active Discovery Targets' set to 192.168.15.1 to 192.168.15.254.  I understand why it is finding the 192.168.2.0/24 subnet, and the devices, and that's great.

But the other subnets are noise or a ghost or something.  How can I hide them?  I tried creating a new zone and was just going to dump them all there, but NetXMS doesn't allow me to move the subnets to the new zone.

Things I tried

1. A filter. Not possible, there are just too many.  If NetXMS had the option to ! (negate) a filter, I could just put !(192.168.15.0/24,192.168.2.0/24) and that could work I guess.  But it doesn't have a negation option.
2. I could use only active scan. I'm trying that now, after having just deleted all the other subnets.  But I do arp monitoring, and I never see addresses for those other subnets, so I don't know why they're appearing in the first place. And honestly I'd rather just leave active + passive on.
3. Move the unwanted ones into a 'Junk' zone. But NetXMS doesn't let me move them.

So what can be done?  Are all these subnets a symptom of some other issue, unrelated to NetXMS?  How can I prevent them from appearing, or hide them?

TIA!

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[tfines]

Update: Even though I switched to active only discovery, and deleted all the unwanted subnets, more are coming back.

[Filipp Sudanov]

Subnets are created on node configuration poll. Possible reason could be some node(s), which have interfaces with periodically changing IP address.
Filtering can be done in Hook::CreateSubnet script in script library, but first of all you can add some debug there (before "return true;" line):

trace(0, "In Hook::CreateSubnet | Conf poll on " .. $node.name .. " want to create subnet " .. $1.ipAddress);

This will add messages to server's log. If you NetXMS version is older then 5.x, line should be a bit different:

trace(0, "In Hook::CreateSubnet | Conf poll on " . $node.name . " wants to create subnet " . $1.ipAddr);