It is possible to specify an address filter in Network Discovery so that it would only scan the necessary subnet you wish to add. You can configure the filter by opening Network Discovery under the Configuration tab in the console and then on the bottom right you will see a section called Address Filters, there you can press Add and specify the address range of the subnet you will be adding.

More on this topic can be found in our administrator guide: https://www.netxms.org/documentation/adminguide/network-discovery.html#address-filters.

Hope this helps!