Shields Up is an online port scanning service created by Steve Gibson of Gibson Research Corporation and hosted at grc.com. The purpose of this utility is to alert the users of any ports that have been opened through their firewalls or through their NAT routers. The utility can scan the most common file sharing ports, as well as all service ports (1-1056), and user defined ports, in sets of 64.
The scanning servers have the static IP addresses of 4.79.142.192 to 4.79.142.207.