018446744073709551615

018446744073709551615 is the decimal representation with a leading zero of the number 18446744073709551615. This value is notable in computing as the maximum unsigned 64-bit integer, equal to 2^64 − 1. In binary it consists of 64 ones, and in hexadecimal it is written as FFFFFFFFFFFFFFFF. The leading zero does not change the underlying value, though in some programming contexts a leading zero can imply octal notation, which could cause confusion since digits 8 and 9 are not valid in octal representations.

As the all-ones value for a 64-bit unsigned integer, 18446744073709551615 is commonly used as a sentinel or placeholder in low-level software. It also serves as a full-bitmask for 64-bit operations, enabling bitwise and masking techniques in systems programming, databases, and networking code. In languages that support unsigned 64-bit types, the same bit pattern represents the maximum stored value; when interpreted as a signed 64-bit integer under two’s complement, the same bit pattern corresponds to −1.

Contextually, this number illustrates the limits of 64-bit architectures and data types, where unsigned storage can hold values up to 2^64 − 1, while signed storage typically ranges from −2^63 to 2^63 − 1. It is commonly cited in documentation as ULLONG_MAX, UINT64_MAX, or similar constants in various programming languages.