511

511 is a natural number that follows 510 and precedes 512. In mathematics it factors as 7 × 73, making it a semiprime. It also equals 2^9 − 1, and in binary it is represented as 111111111. The Roman numeral for 511 is DXI.

In computing and networking, HTTP status code 511 stands for Network Authentication Required. Defined in RFC 6585, this code indicates that the client must authenticate to gain network access, a situation commonly encountered with captive portals on public Wi‑Fi networks.

In North America, 511 is widely used as a travel information telephone number. The service provides real-time traffic conditions, public transit information, and road-closure updates, and is available in many states in the United States and in some Canadian provinces. The 511 program is designed to help travelers make informed routing and timing decisions, reducing congestion and improving reliability on the transportation network.