Web site unreachable via IP address

That sounds like the most likely explanation.
Any particular reason why you want to access the site via IP address?

 

Sometimes, pings are blocked by the firewall at the receiving end.

If you want the IP address do a name server lookup. There's a "lookup" facility in the Network Utility App to do this.

...but several websites can share the same IP address. You can register as many domain names as you like that all point to the same address, on the same server. The web server looks at the domain in the URL and decides which site the user wants.

If you try and put the IP address in the URL, you'll get the "default" site, which may be an actual site (but maybe not the one you wanted), a 'your website goes here' page or an error page.

 

Each web server has an IP. Some sites used name based hosting and share an IP as noted earlier. Others have proxy or load balancers ahead which are the announced IP, but use internal IP's on the actual web servers which can be routable or private. The WAN IP is based on a myriad of network setups and easily identifiable once someone knows the network setup.

So ask the webhost or ISP or "the dude who setup the web server" for the WAN IP or the IP to use to ssh or ftp. They might have a content server that propagates to the production site, or it might be a simple setup with one IP assigned to the server for ssh/ftp, but shared via name based virtual hosting.

And never rely on ping, most admins turns off IMCP for security and other concerns. If a person who knows the IP is not reachable, considering running a traceroute to the web site, it *might* reveal the IP as the last hop, but even this is not 100% reliable.