If you don't want just anybody to play on your server you should consider enabling the Minecraft whitelist system.
Whitelisting can be used to secure your Minecraft server by keeping a list of people allowed to connect to your server. Anybody not on the list isn't able to connect.
This is great if you are running a small server for you and your friends or just want a little more control over who can play on your server.
Please don't rely on keeping the IP address 'secret'. It always gets out and there are people scanning the internet for servers constantly.
First enable whitelisting using the command
whitelist on. You can disable it later using the command
To add a user to the whitelist use
whitelist add PlayerNameHere.
They can be removed using
whitelist remove PlayerNameHere