How do they distinguish between 4 000 000 000.10 and 4 000 000 001.10 wavelengths? Do they simply count how often they cross a full-phase change and keep track

How do they distinguish between 4 000 000 000.10 and 4 000 000 001.10 wavelengths? Do they simply count how often they cross a full-phase change and keep track of the direction of that change?