Friends Can't get on

So, I created a server with the intention of playing some mods with some friends. I am able to join perfectly fine and experience no real issues, but when my friends try to join, they can't. All it tells them is they've been disconnected. I checked to see if whitelist was on, and it wasn't. I had them try again just to be certain and they couldn't, so I enabled whitelist and added their names to the list, and they still can't get on. I gave them the Dyn IP and it still doesn't work for them. I don't know what's going on or why they can't get on. I use the regular IP and it works fine for me, so I don't think it is issues with the IP.