If you are connected to the Internet then your system is vulnerable to outside intruders, even with a firewall in place. Similar to Caller ID on your telephone, CallerIP shows connections to your system and their actual city and country location so you can protect your system before damage is done.
CallerIP tells you the:
IP address, or 'phone number', of someone connecting to your system
city and country location of the IP address
domain registation, including contact information, of the 'owner' of the IP address
network provider being used by the IP address, including contact and abuse reporting information.