I have an Apache web server running on my machine, with a "no-ip" url at joelperl.no-ip.info. I can access the web server from any machine on the internet that I have. I told a friend to go to it, and they said it couldn't be found - DNS error - the same thing you get when you type a website in wrong. I figured maybe No-ip was down or something, so I gave him my real IP address, but the same thing happened. Yes, I asked him to read the IP back to me. It was typed in correctly, and no, he didn't accidentally put a www at the beginning. He could access every other site he tried, but not mine. Why?

check HTTP port!

HTTP port is open. I have no firewall on. My ip address looks valid, it's not one of those MS auto IPs that are private. But if my port was blocked, what error would the web client recieve? It wouldn't be a DNS error, would it? I'd think it'd be a "remote host refused connection" error.

If all of u'r PC's are on the same LAN as the web server, then its u'r IP. Is u'r IP and domain name registered? If u'r friend cant access u'r web server but u can while on the the LAN, then its u'r configuration on the webserver.