What example url that I can use will immediately result in a request failure?

What is the “official” url that I should use if I want to specify only a resource that will work as soon as possible?

I do not want to use www.example.com, as it is the actual site that accepts and responds to requests, and I do not want what is required forever and fails with a timeout (for example, using a random private IP addresses may result in).

I was thinking of writing an invalid address or just some random text, but I decided that it would not look as beautiful and understandable as "www.example.com".