Your logical low will be the Vss voltage (usually found on pin 7 of a 14-pin chip and the logical high will be the Vdd voltage (usually found on pin 14 of a 14-pin chip). My memory is too poor to remember the threshold voltage that differentiates a high from a low .... the average of Vss and Vdd? Can't remember. But don't allow your lows and highs to run outside the boundries of Vss and Vdd. Yes, there are protection diodes that are supposed to stop any damage, but they have their limits and exceeding those voltage at a minimum will slow down the logic.
Dean