One of most difficult things to find (imho) is information about API and Service URL’s for just about any and all applications. Microsoft is better than some (Dell, I am looking at you…) but it can still require some metaphorical google search back flips to get to the right information.
That said… I wanted to quickly document the right mix of URL’s that must be allowed in order for Windows OS license activation to work:
https://go.microsoft.com/
http://go.microsoft.com/
https://login.live.com
https://activation.sls.microsoft.com/
http://crl.microsoft.com/pki/crl/products/MicProSecSerCA_2007-12-04.crl
https://validation.sls.microsoft.com/
https://activation-v2.sls.microsoft.com/
https://validation-v2.sls.microsoft.com/
https://displaycatalog.mp.microsoft.com/
https://licensing.mp.microsoft.com/
https://purchase.mp.microsoft.com/
https://displaycatalog.md.mp.microsoft.com/
https://licensing.md.mp.microsoft.com/
https://purchase.md.mp.microsoft.com/
http://go.microsoft.com/
https://login.live.com
https://activation.sls.microsoft.com/
http://crl.microsoft.com/pki/crl/products/MicProSecSerCA_2007-12-04.crl
https://validation.sls.microsoft.com/
https://activation-v2.sls.microsoft.com/
https://validation-v2.sls.microsoft.com/
https://displaycatalog.mp.microsoft.com/
https://licensing.mp.microsoft.com/
https://purchase.mp.microsoft.com/
https://displaycatalog.md.mp.microsoft.com/
https://licensing.md.mp.microsoft.com/
https://purchase.md.mp.microsoft.com/
If those are blocked for some reason on your network, license activation will fail. I am not sure if all of them are needed for every edition of windows. Server 2012 R2 might required, for example, only a subset of that list vs. Windows 8 or 10 which might require all of them or a different subset.