How to add a page to my website that can only be seen by an administrator?

I have a website (hypothetical) but it doesn't have any authentication. Just people can view it without registering and they can't see any login screen(There is no any Sign-Up, Sign In). But the site has a feature to add posts. That feature can be only used by the Admin. There is a special page designed for him to add posts. But Others should not be able to access that page. How to set up something like that? Without a login? Is it possible?