I go to router panel.
And here Is DHCP section.

So my question is.
IS IT POSSIBLE for user to connect to the router/ use wifi and being invisible here?

[attachment deleted by admin to conserve space]Technically it is yes, however if you have the network set up with WPA2 encryption and use a strong password then it is practically impossible for anyone to get on your network without you giving them the key. The Android APPS you are talking about in your other thread will not let anyone get on your network without your permission.It better be.
But I have found some articles:
http://mods-n-hacks.wonderhowto.com/how-to/crack-wi-fi-passwords-for-beginners-0139793/
http://comorobarwifi.org/hack-wifi-android-and-iphone-apps
...
And some others. So I want to be sure if they dont steal wifi or being invisible?If you are using WPA2 encryption the only practical way to break the password is to keep trying guesses until you get it - If you use something RELATIVELY long and random rather than dictionary words then it would take literally years to be able to crack it which nobody is going to bother doing. See the attached screenshot - With that RANDOMLY generated, 15 character password, the tool at https://www.grc.com/haystack.htm says that it would take "1.49 million centuries" to randomly guess it - So yeah, basically not going to happen! Cracking of WiFi passwords like this simply doesn't happen in the real world.

You also have to bear in mind how far your WiFi network reaches the only people who would be close enough to crack it would be your immediate neighbours or someone sitting outside.

In short, use a decent random password with WPA2 security - You'll have nothing to worry about.

[attachment deleted by admin to conserve space]Thank you camerongray - that is a comforting (and mind-boggling) bit of information.I have security as you said.
But you say neightbors STILL can crack password, how to prevent this?I said they can only crack the password by guessing it, use a secure, random password and you will be fine.Thanks for your patience.