0.0.0.0:80 is a shortcut meaning “bind to all IP addresses this computer supports”. 127.0.0.1:80 makes it bind only to the “lo” or “loopback” interface. If you have just one NIC with just one IP address, you could bind to it explicitly with, say, 192.168.1.1:80 (if 192.168.1.1 was your IP address), or you could list all the IPs your computer responds to, but 0.0.0.0:80 is a shortcut for that.