The legitimacy of port scanning has been challenged in federal court. One such case dates back to the year 2000, in which a dispute between two contractors ended up in a U.S. court because of a dispute over the legality of port scanning.
The plaintiff thought that port scanning was a violation, whereas the defendant believed that just by port scanning was he capable to determine which ports were open and closed on the span of the network he was responsible for. The district court judge ruled that port scanning was legal as long as it did not cause damage.
Does this mean that anyone can scan any and all networks at will? No! Although port scanning is not a violation, you should still try to get permission before scanning a network. Additionally, home users should review their ISP’s terms and conditions before port scanning. Most cable Internet and DSL provider organizations prevent port scanning and maintain the right to disconnect customers who perform such acts, even when they have permission.
In short, It is probably forbidden by law, and probably also by your internet service provider. No one will win a court against you if you don’t do any damage with port scanning and don’t overload the service. Clearly, keyboarding any domain in your address bar would be port scanning too.
