Host Wi-Fi Access Point without tethering
I'm not sure how to provide it with a specific network range however the best way to make an ad-hoc network on your Mac is to disconnect from any other network (as you mentioned you don't want to share a connection you want to make a new one), open System Preferences -> Sharing -> Internet Sharing and share wireless over wireless, it should give you the option to configure your SSID and Security from the same settings page.Hope this helps!