ido-enable-prefix

ido-enable-prefix is a variable defined in `ido.el'.
Its value is

nil

Documentation:
Non-nil means only match if the entered text is a prefix of file name.
This behavior is like the standard Emacs completion.
If nil, match if the entered text is an arbitrary substring.
Value can be toggled within Ido using `ido-toggle-prefix'.

You can customize this variable.