1.7, 1.8 and Maybe 1.9?

Hey I noticed many servers can use 1.7 1.8 and 1.9? can somebody tell me a detailed tutorial on how to do this? Im really not the greatest with this kind of stuff and I looked everywhere and I couldn't find out how to do this

 

They use different server software, like Spigot or SpongePowered.

 

@Boggle_Poop right, so what happends when Minecraft uploades is thst the code changes and new things are added or removed. This means that the servers need to update too so they are able to communicate with the Minecraft version and show the player change such as horses etc. that means that if any 1.7 client tries to join a 1.8 server, the server will talk to it on a different language that it can understand, in other words it will tell the client to do things it doesn't know about.
So what do they do in the so called "Protocol hacked" servers?
Well they make the server talk both languages so that it can talk to 1.7 and 1.8 clients, so when a client joins the server they check what version it runs on and they talk to it appropriately sending the correct packets with the correct values. That's why when we had the 1.7 and 1.8 version, if an 1.8 client tried to place a block thst didn't exist in 1.7, it would turn into stone.
That's why usually they adapt older versions to newer clients like a 1.7 server to work with 1.8 clients, and that has many drawbacks such as flaws that can prove fatal for runtime, and also that the latest clients cannot use the latest features in this server...
There is no configuration you can change to change the versions accepted in the server really.... You need to change the code to adapt and that is not an easy task.