Check Who's Connected To The Router, Scan Your Network With "Angry IP Scanner"

Angry IP scanner is free and open-source (IP and port scanner) tool released under GPLv2 license. It scans the network and detect attached details of those devices that are connected to the router. It displays details like IP address, Port address, MAC address, NetBIOS information and so on. The amount of gathered data about each host can be extended with plugins. It can scan IP addresses in any range as well as any their ports. It is cross-platform and lightweight, it can be freely copied and used anywhere. It also has additional features, like NetBIOS information (computer name, workgroup name, and currently logged in Windows user), favorite IP address ranges, web server detection, customizable openers, etc.
Scanning results can be saved to CSV, TXT, XML or IP-Port list files. With help of plugins, Angry IP Scanner can gather any information about scanned IPs. Anybody who can write Java code is able to write plugins and extend functionality of Angry IP Scanner.
In order to increase scanning speed, it uses multi-threaded approach: a separate scanning thread is created for each scanned IP address. You can also download source code of this program or download program for other operating systems from this download page.

To install Angry IP Scanner in Debian/Ubuntu/Linux Mint/Ubuntu derivatives open Terminal (Press Ctrl+Alt+T) and copy the following commands in the Terminal:
>>> For 32bit

Terminal Commands:
wget -O ipscan_3.3.1_i386.deb http://github.com/angryziber/ipscan/releases/download/3.3.1/ipscan_3.3.1_i386.deb
sudo dpkg -i ipscan_3.3.1_i386.deb
rm ipscan_3.3.1_i386.deb

>>> For 64bit

Terminal Commands:
wget -O ipscan_3.3.1_amd64.deb http://github.com/angryziber/ipscan/releases/download/3.3.1/ipscan_3.3.1_amd64.deb
sudo dpkg -i ipscan_3.3.1_amd64.deb
rm ipscan_3.3.1_amd64.deb

That's it