'Can't Reach Server' problem

[JONESY-9931]

Hey guys. when i have made my server and my brother try's to join me or my mate it says to them 'cant reach server'. i'm not sure why? Also if they click refresh it still says 'cant reach server'. then when they click join server it still doesn't work???

But it lets me join there server sometimes.

If anyone knows why, or something what might help. could i get advice or comments to help me out ......that would be great... Thanks...