IPv4 and IPv6 Explained: Addressing, Routing, and Monitoring
IPv4 and IPv6 are the two IP versions that move packets across modern networks, and routing is what decides where those packets go next. If a website or API works over IPv4 but fails over IPv6, or if traffic takes a bad path in one region and not another, the problem is often not the application itself but addressing, DNS, or routing behavior underneath it.
What IPv4, IPv6, and routing actually mean
IPv4 is the older and still extremely common internet protocol based on 32-bit addresses such as 203.0.113.10. IPv6 is the newer protocol with a much larger address space and addresses like 2001:db8::10. Both are used to identify hosts and move traffic between networks, but they are distinct stacks with different operational behavior.
Routing is the process routers use to decide where packets should go next. Your device usually does not know the full path to a destination. It sends traffic to the next hop, and each router along the way makes another forwarding decision until the packets reach the target.
When IPv4, IPv6, or routing differences matter
A website works for some users but not for others
One region, ISP, or client stack may prefer IPv6 while another falls back to IPv4. That can create split behavior even when the application is unchanged.
DNS returns both A and AAAA records
If a domain publishes both IPv4 and IPv6 answers, clients may try one path first. A broken AAAA path can hurt users even if IPv4 is healthy.
A firewall or route is only correct for one IP version
It is common to harden or troubleshoot IPv4 and accidentally forget the equivalent IPv6 path, ACL, or listener.
You need to understand why monitoring from one place is not enough
Routing can differ by geography and by IP version, which is why multi-location monitoring gives a more realistic picture of user experience.
How addressing and routing fit together
DNS tells the client which IP address to use
For IPv4 this is usually an A record. For IPv6 it is usually an AAAA record. The available answers influence which network path the client attempts first.
The client sends traffic to the next hop
Your device does not calculate the whole internet path itself. It sends packets toward a gateway or router that knows how to forward them further.
Routers forward packets hop by hop
Each router makes a decision based on its routing table until the traffic reaches the target network and then the target host.
If one IP version or route is broken, users can still see an outage
A service may be healthy at the application layer but still fail for users whose clients choose a broken IP version, DNS answer, or routing path.
How to think about common IP and routing concepts
These are a few practical concepts that matter when diagnosing IPv4, IPv6, and routing behavior:
IPv4 address 203.0.113.10 IPv6 address 2001:db8::10 A record example.com -> 203.0.113.10 AAAA record example.com -> 2001:db8::10 Private IPv4 192.168.1.10 or 10.0.0.10 Next hop The router that forwards the packet onward
IPv4 and private addressing
IPv4 often relies on private address space and NAT in local networks because globally routable IPv4 addresses are limited.
IPv6 global reachability
IPv6 was designed to remove address scarcity and make large-scale direct addressing easier, but it still requires correct routing and firewall policy.
A vs AAAA records
A records publish IPv4 destinations. AAAA records publish IPv6 destinations. A domain can serve one, the other, or both.
Routing path differences
IPv4 and IPv6 traffic may take different network paths, so latency, packet loss, and availability can differ between the two.
Why IPv6 problems are easy to miss
A service can look healthy during manual testing if your own network path uses IPv4, while a subset of users fails because their clients prefer IPv6 and hit a broken route, bad firewall rule, or wrong AAAA record.
That is why dual-stack services deserve explicit monitoring. A healthy IPv4 endpoint does not automatically prove that IPv6 users have the same experience.
IPv4 vs IPv6
| Topic | IPv4 | IPv6 |
|---|---|---|
| Address style | 32-bit addresses such as 203.0.113.10. | 128-bit addresses such as 2001:db8::10. |
| Common operational reality | Often uses NAT and private address ranges inside local networks. | Designed for massive address space and broader direct addressing. |
| Monitoring implication | Can look healthy even when the IPv6 path is broken. | Can fail for real users even when IPv4 testing still passes. |
Need to verify that DNS is returning the right A or AAAA records?
DNS monitoring helps confirm that domains still resolve to the IP addresses you expect, which is critical when IPv4 and IPv6 paths behave differently.
Read the DNS monitoring guide βHow to interpret common scenarios
The site works over IPv4 but fails over IPv6
This usually points to a broken AAAA record, missing listener, bad IPv6 firewall policy, or an IPv6-specific routing problem rather than a total application outage.
Different regions see different latency
Routing is not identical everywhere. Traffic from different locations can take different providers, peering paths, or transit routes.
Ping works, but only for one IP version
That means the basic path may be healthy for one stack and broken for the other, which is a strong signal to compare A vs AAAA behavior directly.
A domain suddenly becomes unstable after a DNS change
Publishing or changing an AAAA record can expose IPv6 users to a path that was never fully tested in production.
Important limitations
- β A general routing explainer does not replace protocol-specific debugging for BGP, ACLs, or advanced network design.
- β A successful test from one source network does not prove every region or ISP sees the same route.
- β DNS, routing, firewall policy, and application listeners can all interact, so one failing symptom may have multiple underlying causes.
- β A healthy hostname over one IP version does not prove the other IP version is correctly configured.
How people test IPv4, IPv6, and routing manually
dig example.com A Useful for checking which IPv4 address a domain currently returns.
dig example.com AAAA Useful for confirming whether a domain publishes an IPv6 destination at all.
traceroute6 example.com Helpful when you want to see whether the IPv6 path behaves differently from the IPv4 path.
Frequently asked questions
Your service is only as reachable as the IP path users actually take.
nsmon helps you monitor DNS, reachability, latency, and service behavior from multiple probe locations so you can catch IPv4, IPv6, and routing problems before they turn into user-visible outages. Create a free account to start monitoring from the perspective real networks use.