When running KMS there may problems getting some clients to find the KMS server for license validation. There can be many reasons for this but if you know traffic is allowed between the client and the server and it still doesn't work you can specify the KMS server manually. slmgr.vbs /skms <value>:<port> The port is only needed when using a non-standard port. After the server has been specified Windows can be activated as usual. I have needed this many times while deploying Windows servers in a DMZ zone.