Cracks
This page is available to address users of Minecraft Cracks.
Minecraft cracks are illegal software piracy.
Obviously right off the bat: don't do crimes. Just don't.
Secondarily: cracked minecraft clients and offline-mode servers both are modified from their original functionality and limited from using the full functionality of minecraft. As such, even if we chose to support illegal players/servers, we would be unable to as many troubles encountered are the incurable fault of not using the official client on an online-mode server.
As a bonus down-side to this: many cracked clients contain malware, infecting the PC of anyone who installs it. (what else could you expect when downloading illegal software cracks?)
NEVER EVER EVER turn off "online-mode" in server.properties - the exception is when you're using a Bungee server, which handles the online-mode functionality from the Bungee proxy (if you do this: be sure to firewall off external access to your server except through the Bungee port).
It is important to be aware that there is no such thing as a "premium account" in minecraft. You either have an account (a validly registered and purchased Mojang account), or you don't. If you don't have an account and you're playing minecraft: you're running a cracked minecraft client.