A free program for Android, by Magdalm.

Who’s on my wifi takes WiFi security to the next level. Scan your local network and learn who uses it, what they do, and which devices are connected to it. With this app, you can keep an eye on all devices connected to your home network by showing you which ports they open or through the day/night mode. You can keep track of who’s out of the house and when. It can show you devices connected and disconnected from your network, their hostnames, IP addresses, and gateway.

Who’s on my wifi is your go-to app for managing your local network. You can even add a widget of the app on your home screen for easy access.