.com

With roughly 160 million registrations, .com is the largest top-level domain in existence and the default extension for commercial activity online. VeriSign has operated the registry since the domain's delegation in 1985, when it was one of the original generic extensions set aside for commercial entities. That original intent never hardened into policy: registration is open to anyone, anywhere, with no eligibility screening, and the namespace now serves businesses, brands, and private individuals indiscriminately. The extension's ubiquity gives it a recognition advantage no rival has matched, and it also explains why short unregistered strings under .com have become exceptionally scarce.