As I understand it, the full address would be http://www.<site name> That tells you it is a World Wide Web site and it is using Hypertext Transfer Protocol. I think that your browser assumes this and fills in the missing bit, because I have just tried both on AO and it takes me to the site, having completed the address box in full, in the navigation bar.

I suspect that if it is a private/internal site, it is just http://?

Other possibilities that spring to mind are ftp:// which is File Transfer Protocol, and the addition of an "s" (https:// etc) which indicates a secure site.

I am sorry that I am no expert, my comments are just based on my casual observations