Is there a way to select multipal IP's (not in a the same subnet) and Tag them?
Yes you can create a static tag and drag that onto the IP addresses that you wish to give that property, or you can create dynamic tag criteria and each host that matches will inherit that tag.
Is there a more specific use case you had in mind please ?
When I need to do that, I do an 'Asset Search' for the specific IP's, then in the 'Asset Search Results' screen I select all ip's and click on 'create tag'.
Hope that helps.
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