These are the generic domain extensions used on the Internet and were the first to appear. They were originally designed to trade (.com), networks (.net) and organizations (.org), but nowadays they do not have (like the rest of generic extensions) any requirements for granting them and they can be requested by anyone.

To prevent the use of one´s name by others on the Internet, one usually registers a domain at least at each of these three extensions, even if one uses only of them initially.