cant test url with ip

I have deploy the webpagetest in my vm.
but in some machine when i test website with ip,it allways tell me that"you can not test from the public internet. your web site needs to be hosted on the public internet for testing".In this machine when i test some website with domain name it works ok.And in some machine it can test url with ip.
I dont know whats the difference between them.And whats the role?How can i solve.

Thanks in advance

There is a setting in settings/settings.ini on the server to allow for testing of private networks. You juts need to enable it: