Why can’t my computer open site B?

The inability to access a specific website, such as "site B," is almost always a connectivity or configuration issue localized to your machine or its immediate network path, rather than a problem with the site itself being universally offline. The most probable causes involve DNS resolution failures, local firewall or security software interference, or corrupted network state data on your computer. A DNS failure occurs when your computer cannot translate the human-readable domain name (e.g., www.siteb.com) into the numerical IP address required to route the request; this can stem from misconfigured network settings, issues with your Internet Service Provider's DNS servers, or a corrupted local DNS cache. Simultaneously, overzealous antivirus suites or built-in firewall rules can mistakenly classify a site as a threat and silently block the connection, while problems with your browser—such as an out-of-date SSL certificate store, a conflicting extension, or corrupted cookies and cache for that specific site—can also cause a failure to load.

To diagnose this systematically, you must isolate the point of failure. Begin by checking the site's availability from a different network, such as using your smartphone on cellular data instead of your Wi-Fi, or by asking someone else to try accessing it. If the site loads elsewhere, the problem is confined to your local system or home network. The next step is to use command-line tools to trace the network path. Running `ping siteb.com` tests basic reachability but may be blocked by some servers; a more informative test is `nslookup siteb.com` or `dig siteb.com`, which will show if your computer is receiving a valid IP address from DNS. If these commands fail or return an error, the issue is DNS-related. You can then flush your local DNS cache (using `ipconfig /flushdns` on Windows or `sudo dscacheutil -flushcache` on macOS) and temporarily switch your DNS server to a public one like Google's (8.8.8.8) or Cloudflare's (1.1.1.1) to see if the problem resolves.

If DNS is functioning and the site remains inaccessible, the blockage is likely at the application level. Try accessing "site B" using a different web browser or a private/incognito window, which disables extensions and isolates your session. A successful load here implicates your primary browser's extensions, cache, or settings. Should it still fail, your local firewall or security software is the prime suspect. Temporarily disable these protections as a test—while being mindful of security—to see if the connection proceeds. For advanced users, the `tracert` (Windows) or `traceroute` (macOS/Linux) command can reveal if the network request is being dropped at a specific hop, potentially indicating a routing issue with your ISP. Furthermore, check if the site uses HTTPS; an expired or mismatched SSL certificate on the site can cause browsers to reject the connection, though this typically produces a specific warning message rather than a complete failure to open.

Ultimately, resolving this requires methodical elimination. The sequence should be: verify external accessibility, test DNS, inspect browser state, and then scrutinize local security software and network routing. Persistent failure across all devices on your local network would point to a restriction at the router level—such as parental controls or IP filtering—or an ISP-level block. In rare cases, the site's IP address might be blocked by your router's firewall or your computer's hosts file, a local override file that can redirect or block domain names. Accessing "site B" is typically restored by correcting the DNS configuration, clearing your browser's cached data for that site, or adding an exception for it in your security software's rule set.