The Special:RangeContributions page should have an interface that lets you enter a list of IP addresses and then it gives you the smallest CIDR range that includes all of them. It should support both IPv4 and IPv6.
The Interface should probably be collapsible at the top of the form.
If there are subranges, it should list them. See http://nativeforeigner.com/calc/.
All result ranges should be clickable, which should submit them for a search at Special:RangeContributions.