If you think someone is using your Wi-Fi network without permission, you can confirm by scanning for connected devices on your Wi-Fi network. Follow these directions.
Open and unsecured ports can be a serious security risk on Linux systems. Use NMAP to discover open ports on your Linux system and secure them.
nmap is a powerful tool for network discovery. There is a graphical front-end for nmap called zenmap, which makes nmap much easier to use.
Following on from the first part in this series, it is time to look at some of the more advanced features of nmap.
nmap is a network scanning tool which can be used for a whole variety of network discovery tasks including port scanning, service enumeration and OS fingerprinting.