What Is a CDN Connectivity Test?
This tool sends genuine, real HTTP requests directly from your browser to several major content delivery networks (CDNs) - the services that host copies of popular scripts, fonts, and libraries for websites all over the internet - and reports whether each one is actually reachable and how long the request took. If a website you're using looks broken or half-loaded, a blocked CDN is a common but easy-to-miss cause, since the rest of the page can load fine while just its CDN-hosted assets fail silently.
How to Use the CDN Connectivity Test
- Select 1 quick round for a light test or 3 rounds to compare response-time consistency, then select a timeout.
- Click Start Test. Your browser makes real requests directly to cdnjs, jsDelivr, unpkg and Google Fonts. Each result is shown separately; requests download a small public asset and may use mobile data.
- Look for Verified when the browser can read HTTP 200 and the expected asset content. Unverified means a cross-origin opaque response prevented HTTP/content checks, not a confirmed failure. Failed means an HTTP error, unexpected content, timeout or browser request error.
- Compare each provider's verified-request count and median HTTP completion time. The chart draws only verified measurements; it does not claim to measure your network's DNS time or the CDN's global performance.
- Use Stop to cancel ongoing requests, Export CSV to save the observed request details and Reset to clear this tab's session data. Repeat the test on another network if results differ.
How It Works
This tool sends a real HTTP request to a genuine, commonly-used file hosted on each CDN - a well-known JavaScript library file for the script-hosting CDNs, and a real font stylesheet request for Google Fonts - timing precisely from request start to response. A successful response confirms the CDN is genuinely reachable from your current network; a network error or timeout is reported as unreachable, which most commonly indicates that specific domain is being blocked somewhere along your network path.
Understanding Your Results
- All CDNs show "Reachable" - your network has no restrictions on these common content delivery services, which is the expected result for most home and typical business networks.
- One or two CDNs unreachable, others fine - some networks block specific domains individually, often due to a corporate firewall's specific denylist rather than blocking CDN traffic broadly.
- All CDNs unreachable - could indicate a broader network restriction, an overly-aggressive firewall or content filter, or in rare cases, general connectivity problems worth checking separately.
- Reachable but with notably high response times - reflects normal latency to that CDN's servers based on your geographic location, not necessarily a blocking issue.
Browser Limitations
This test uses "no-cors" mode for the actual reachability checks, which means it can confirm whether a request reaches the server successfully but can't inspect the exact response content - sufficient for a genuine reachability check without needing full access to the response body. A CDN blocked by very selective content filtering that still allows the connection but strips specific content might not always be caught by a basic reachability check like this one.
Common Mistakes to Avoid
- Assuming a website itself is broken when a CDN is blocked - many sites rely on third-party CDNs for scripts and fonts, so a blocked CDN can make an otherwise fine website appear broken or unstyled.
- Testing only once and generalizing to all CDNs - different networks block different specific CDN domains, so results can vary meaningfully between the services tested here.
- Confusing a slow response with a blocked one - a genuinely slow but eventually successful response is different from an outright blocked or unreachable connection.
Frequently Asked Questions
Why would a website look broken if a CDN is blocked?
Many websites load JavaScript libraries, icon fonts, or web fonts directly from CDNs rather than hosting them themselves; if that CDN is blocked, those specific resources fail to load while the rest of the page may still function.
Why are some CDNs blocked on certain networks?
Corporate and school networks sometimes block specific CDN domains as part of broader content filtering or security policies, occasionally as an unintended side effect of blocking a category that CDN domain happens to fall into.
Is any of my personal data sent during this test?
No, this test only requests a fixed, publicly available file from each CDN to check reachability - no personal data is included in these requests.
Why does this test use "no-cors" mode?
This mode allows the test to confirm a request reaches its destination successfully without requiring full cross-origin response access, which is sufficient for a genuine reachability check.
Can I fix a blocked CDN myself?
On a corporate or school network, this typically requires a network administrator to adjust firewall rules; on a home network, checking your router's content filtering settings can sometimes reveal the cause.
Conclusion
If a website seems broken or unstyled, checking whether its likely CDN is reachable here can quickly clarify whether that's the actual cause. For related connectivity checks, pair this with the DNS over HTTPS Availability Check.