IPv4 vs IPv6
Every device on the internet needs an IP address. IPv4 — the original protocol — has run out of addresses. IPv6 is the replacement, with vastly more address space. Most modern networks run both simultaneously. Here's what this means for speed and your connection.
IPv4 vs IPv6 — key differences
| Property | IPv4 | IPv6 |
|---|---|---|
| Address format | 192.168.1.1 | 2001:db8::1 |
| Address space | ~4.3 billion addresses | 340 undecillion addresses |
| Address length | 32 bits | 128 bits |
| NAT required | Yes — most home devices share one public IP | No — each device can have a unique public address |
| Header size | 20–60 bytes (variable) | 40 bytes (fixed) |
| Built-in security | Optional (IPSec) | IPSec support built into spec |
| Router fragmentation | Allowed | Not permitted — handled end-to-end |
| Deployment status | Universal — all networks support IPv4 | ~45% global internet traffic as of 2024 |
Why IPv4 addresses ran out
IPv4 uses 32-bit addresses, giving a theoretical maximum of ~4.3 billion unique addresses. In the 1980s when IPv4 was designed, this seemed inexhaustible. The explosive growth of internet-connected devices — smartphones, tablets, IoT devices, smart TVs — consumed the available pool. The last large blocks of IPv4 addresses were exhausted by IANA (the internet's address authority) in 2011.
The workaround is NAT (Network Address Translation): your router holds one public IPv4 address from your ISP, and all devices in your home share it. Internally, devices use private address ranges (192.168.x.x, 10.x.x.x) that are translated at your router. This works for most consumer use, but adds complexity and breaks some peer-to-peer applications.
Does IPv6 affect your internet speed?
For most users, IPv6 has no perceptible impact on speed. There are marginal theoretical advantages:
- No NAT overhead: IPv6 devices connect directly, removing the router's NAT translation step. For high-throughput connections with many simultaneous connections, this reduces CPU load on the router
- Fixed-length headers: Routers process IPv6 headers more efficiently than the variable-length IPv4 headers
- Better path selection: Many ISPs and CDNs prefer IPv6 paths which may have lower congestion in some regions
In practice, these differences are in the milliseconds-or-less range and are undetectable for normal use.
Dual-stack: how most connections work today
Modern internet connections use dual-stack — your router gets both an IPv4 address and an IPv6 address from your ISP, and your devices can use both. When you connect to a website, your operating system's Happy Eyeballs algorithm (RFC 6555) tries both IPv4 and IPv6 simultaneously and uses whichever connects first. If a site supports IPv6, your device will typically use it; if not, it falls back to IPv4.
This is transparent to the user. You don't need to configure anything to benefit from IPv6 on a modern router.
What to check if IPv6 is causing problems
In some cases, IPv6 misconfiguration can cause connection problems:
- Slow initial page loads: If your ISP provides IPv6 but it's poorly configured, Happy Eyeballs may attempt IPv6 first, fail (or take too long), then fall back to IPv4. This adds 300–500 ms to initial connections. Disabling IPv6 at the router level is the temporary fix; the real fix is ISP-side
- VPN IPv6 leaks: Some VPNs route IPv4 traffic through the tunnel but leave IPv6 traffic unrouted, revealing your real IPv6 address. Check that your VPN explicitly handles IPv6, or disable IPv6 in network settings when connected to the VPN
You can test whether your connection is using IPv4 or IPv6 — and whether both are working — using the IP test tool on this site.
Privacy: IPv6 address tracking
With IPv4 and NAT, all devices in your home share one public IP address, making individual device tracking harder. With IPv6, each device can have its own globally unique address, potentially making per-device tracking easier. Privacy Extensions (RFC 4941) address this: they generate temporary, random IPv6 addresses for outgoing connections, regenerated periodically. These are enabled by default on Windows, macOS, and iOS. How DNS and IP addresses work together →