I think it's because your friend has to make a hamachi network, and you need to join it.
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Type in the IP address or Hamachi address of your friend's correctly or play the premium Minecraft. Myncraft1: Your friends has to be on (Hamachi) to connect to his server
Well there are many ways of setting up a minecraft server. Some people use the basic method to just use the Minecraft Server Application found on The website of Minecraft. But for an advanced server where you can use plugins you have to use CraftBukkit. To find it go on Bukkit.Org. For the IP for others to connect is a bit complicated. The easiest way which even i use is to download Hamachi. When you install it. Hamachi will give u a generated ip. Put that Ip in to ur Minecraft Server.properties. The IP there. Then you and your server friends have to use that IP to connect. And Hamachi has to be on for it to connect.
You can add friends on any server with a new Minecraft plugin called FriendsForMinecraft available at the related links below.
Maybe the server is temporarily not available or you are using anjocaido's minecraft launcher. If you have that you need to download Hamachi and Type Server iD IP Password and Gate
Use the hamachi server.