Connection problem with friends

Hello, I hosted a server and my friends connected normaly. But when I send my server to my friend with better computer I could connect to him, but the other friend (from another city) couldn't. Why can't he connect to the server ? Is it range problem ?
 
Let's clarify the connections for such a typical setup

  1. you have an ISP
  2. and it assigns an IP address to the router, sets the DNS settings
  3. you create a server on some PC (say address x.100 and it needs to be always the same address)
  • your friend's pc has some address
  • is connected to his router
  • and to his ISP
So how does he get to your pc at x.100? Secondly, what port(s) is/are required - - do you know?
 
I forgot to say it's a Minecraft Bukkit game server.
I told my friend about hosting a game server using port forwarding.
When I was hosting a server before him, both He and his friend can connect normaly to my server.
But when he start hosting (Our)server with his IP, I connected normaly, but his friend was unable to connect, probably because he is living far from us, but I am not sure.
 
Distance is not the issue, but the network settings are of the server system are.
Also, the PUBLIC address of the server system (not the lan address) must be known and given to all players.
 
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