If you create an IP location per subnet you could run the following report will give you a count of the amount of assets in an existing IP location. More information about IP locations can be found in this knowledgebase article: https://www.lansweeper.com/kb/117/grouping-assets.html
Select Top 1000000 tsysIPLocations.IPLocation,
Count(tblAssets.AssetID) As CountAssets
From tblAssets
Right Join tsysIPLocations On tsysIPLocations.LocationID =
tblAssets.LocationID
Group By tsysIPLocations.IPLocation