IPv4 vs IPv6: Why Do Some Websites Work and Others Don't?

Sometimes a website loads perfectly on your phone but not on your laptop. Sometimes it works on Wi-Fi but fails on mobile data. Sometimes a domain works for one person, while another gets a timeout, a broken SSL error, or a page that never finishes loading.

One common reason is the difference between IPv4 and IPv6. Modern networks often support both, but not every website, DNS provider, router, firewall, VPN or hosting platform handles both protocols correctly.

IPv4 and IPv6 in Simple Terms

IPv4 and IPv6 are two versions of the Internet Protocol. They both let devices communicate over the internet, but they use different address formats and sometimes follow different network paths.

• IPv4 addresses look like 203.0.113.10.
• IPv6 addresses look like 2001:db8::1.

IPv4 is older and still widely used. IPv6 was created because the internet needed many more addresses. Many internet connections today are dual-stack, which means they can use both IPv4 and IPv6.

That sounds simple, but dual-stack also means there are two possible ways to reach the same website. If one path is misconfigured, the website may work for some users and fail for others.

Why a Website May Work on IPv4 but Fail on IPv6

A website can publish both an IPv4 DNS record and an IPv6 DNS record. The IPv4 record is usually an A record. The IPv6 record is an AAAA record.

If the A record points to a working server but the AAAA record points to a broken server, users who prefer IPv6 may fail while IPv4-only users succeed.

Common causes include:

• The domain has an AAAA record, but the server is not listening on IPv6.
• The IPv6 address points to the wrong server.
• The hosting provider supports IPv4 but not IPv6 on that plan.
• The firewall allows IPv4 traffic but blocks IPv6 traffic.
• The SSL certificate or virtual host is configured only for the IPv4 path.

This is why a site can look healthy in one test but still fail for real users. The IPv4 route may be fine while the IPv6 route is broken.

Why a Website May Work on IPv6 but Fail on IPv4

The opposite can also happen. Some networks now rely heavily on IPv6, especially mobile networks and newer ISP deployments. If a website or service only works properly over IPv6, IPv4 users may experience problems.

This can happen when:

• The IPv4 address is behind CGNAT or an overloaded NAT gateway.
• The IPv4 firewall rules are stricter than the IPv6 rules.
• The IPv4 DNS record points to an old server.
• The IPv4 route between the user and the host is degraded.
• The site owner updated IPv6 but forgot to update IPv4.

IPv6 is not automatically more reliable than IPv4, but it can sometimes avoid IPv4-specific issues such as NAT exhaustion, port forwarding limitations or legacy routing problems.

DNS Records Are Often the First Place to Check

When a browser opens a website, it usually asks DNS where the domain points. If the domain has both A and AAAA records, the browser may try IPv6 first, IPv4 first, or race both depending on the operating system and network behavior.

If DNS is wrong, everything after that becomes confusing. You may see timeouts, intermittent loading, SSL errors, or different results from different locations.

For troubleshooting, check:

• Does the domain have an A record?
• Does the domain have an AAAA record?
• Do both records point to the correct infrastructure?
• Do DNS resolvers around the world return the same answer?
• Was a DNS change made recently and still propagating?

You can use the DNS Propagation Checker to compare DNS answers from multiple resolvers and spot inconsistent records.

Firewalls Often Treat IPv4 and IPv6 Separately

A very common mistake is securing IPv4 but forgetting IPv6, or allowing IPv4 but blocking IPv6 by accident. Many firewall systems maintain separate rule sets for each protocol.

For example, a server may allow port 443 over IPv4 but silently drop port 443 over IPv6. From the user's perspective, the website simply does not load. From the administrator's perspective, everything may look fine if they only tested IPv4.

If you host a website, API, VPN server, game server or self-hosted app, test both protocols when possible. Also check whether your cloud security groups, server firewall, CDN, reverse proxy and web server all support the same protocol.

VPNs Can Change the Result

A VPN can make IPv4 and IPv6 behavior even more confusing. Some VPNs support IPv4 only. Some support IPv6 fully. Some block IPv6 to prevent leaks. Others accidentally allow IPv6 traffic outside the tunnel.

That means a website may work without the VPN, fail with the VPN, or reveal a different IP version than expected.

Common VPN-related causes include:

• The VPN tunnels IPv4 but not IPv6.
• IPv6 is blocked by the VPN client.
• The website blocks known VPN or datacenter IP ranges.
• DNS requests go through one path while web traffic goes through another.
• The VPN exit server has poor IPv6 routing.

If the problem only appears while using a VPN, test your connection with the VPN Check, then verify whether DNS or WebRTC exposes unexpected addresses.

Routing Problems Can Affect Only One Protocol

IPv4 and IPv6 traffic do not always take the same path across the internet. A routing issue may affect one protocol but not the other.

This is why a site can fail from one ISP but work from another. It can also explain why mobile data works while home Wi-Fi does not, or why users in one country report issues while others do not.

When the DNS records are correct and the server is configured properly, routing is the next suspect. A traceroute can help reveal where the path slows down or stops.

Use Traceroute to inspect the route between your connection and a target host. It will not solve every routing problem, but it can show whether traffic is failing close to your network, your ISP, a transit provider, or the destination.

How to Troubleshoot IPv4 vs IPv6 Problems

Use a simple checklist instead of guessing:

1. Check your public IP. See whether your current network uses IPv4, IPv6, or both.
2. Check DNS records. Look for A and AAAA records and compare answers across resolvers.
3. Test without VPN. Then test again with VPN enabled.
4. Try another network. Compare home Wi-Fi, mobile data and another ISP if possible.
5. Check ports. Confirm that HTTP, HTTPS or the service port is reachable.
6. Inspect firewall rules. Make sure IPv4 and IPv6 are both allowed where needed.
7. Run traceroute. Compare the path when the site works and when it fails.

If you manage the website, do not stop after testing from your own laptop. Test from an IPv4-only network, an IPv6-capable network, mobile data and a VPN. Real users do not all reach your site through the same path.

Should You Disable IPv6?

Disabling IPv6 can sometimes make a broken website load, but it is usually a workaround, not a real fix. IPv6 adoption continues to grow, and many mobile networks already rely on it heavily.

If you are a user, disabling IPv6 may help confirm that the problem is IPv6-related. If you are a site owner, the better fix is to configure IPv6 correctly or remove broken AAAA records until your IPv6 service is ready.

For VPN users, disabling IPv6 may reduce leak risk if the VPN does not support IPv6. But the ideal setup is a VPN that handles IPv4, IPv6 and DNS consistently.

Conclusion

When some websites work and others do not, the problem is not always your browser. IPv4 and IPv6 can expose hidden differences in DNS, routing, firewall rules, VPN behavior, hosting configuration and server support.

The practical approach is to test both paths. Check your IP version, inspect DNS records, verify ports, compare VPN and non-VPN behavior, and use traceroute when the route itself may be failing.

What to Read Next

• CGNAT Explained: What Carrier Grade NAT Is and Why It Matters
• WHOIS vs DNS Lookup: What's the Difference?
• How to Prevent IP Leaks While Using a VPN