Prefix with a single quote so that the value will be interpreted as text.
Excel and almost all other software uses IEEE standard DP double precision
(DP) to store numbers. DP can represent all 15 digit numbers, but only some
16 digit numbers. Rather than explain why numbers with more than 15 digits
changed on entry to other numbers with more than 15 digits, MS decided to
only display 15 digits, as documented in Help.
When you enter a 17 digit number in a cell, it is truncated (not rounded) to
15 digits before conversion to binary.
Jerry