It's a normal URL, but with an IP address in the domain part. Your browser normally uses a DNS server to resolve the domain name portion of a URL to the hostin

It's a normal URL, but with an IP address in the domain part. Your browser normally uses a DNS server to resolve the domain name portion of a URL to the hosting server's IP address — news.ycombinator.com resolves to 174.132.225.106, for example — but it skips that step if it sees something that looks like an IP address itself. (In either case, it tells the server what the original hostname was, so that it's possible to host more than one website at a single IP address.)