sudo apt-get install arpwatch
edit it's config:
sudo nano /etc/default/arpwatch
use the following line:
ARGS="-N -p -i eth0 -m email_adress@email_server.tld"
- eth0 is your internal network interface
- email_adress@email_server.tld is the email adress where you want to be notified. you have to be able to send emails from your server. you can send emails without email server.
you can restart, start and stop the application by using:
sudo nano /etc/init.d/arpwatch restart
for more info read the manual: