---
title: IPv4 and IPv6 checks
description: Choose whether a monitor is checked over IPv4, IPv6, or whichever answers.
category: configuration
updated: 2026-07-23
tags: [ipv6, ipv4, dns, configuration]
---

Every monitor has an **IP Version** setting. It decides which kind of address we use
when we check your service.

![The IP Version selector in the Add Monitor dialog offering Automatic, IPv4 only and IPv6 only, with a note explaining that Automatic uses whichever address the host answers on](/img/docs/app-ip-version.png)
*Automatic is the default. Pick **IPv6 only** when you specifically want to prove
IPv6 works - it fails instead of quietly falling back.*

## The three choices

**Automatic** (the default) — we use whichever address your host answers on. If your
domain has both an IPv4 (`A`) and an IPv6 (`AAAA`) record, we try one and fall back to
the other, so a monitor stays up as long as *either* works. This is what you want for
most monitors.

**IPv4 only** — we check the `A` record and nothing else.

**IPv6 only** — we check the `AAAA` record and nothing else. If your host has no IPv6
address, or IPv6 is broken while IPv4 is fine, the check **fails** rather than quietly
falling back. That's the point: it's the only way to prove your service really is
reachable over IPv6.

## Why pin to IPv6

Automatic mode hides IPv6 problems. If your `AAAA` record points somewhere broken, an
automatic check falls back to IPv4, reports "up", and you never find out — while
IPv6-only visitors (increasingly common on mobile networks) can't reach you at all.

A common setup is two monitors on the same host: one **IPv4 only** and one **IPv6
only**. You then see exactly which side of your stack is unhappy, instead of one
monitor that goes green as long as anything answers.

## What you'll see when it fails

If you pin a monitor to a family your host doesn't publish, the check reports a clear
reason — for example *"example.com has no usable IPv6 (AAAA) address, but this monitor
is pinned to IPv6"* — rather than a vague timeout. That message usually means the
`AAAA` record is missing, not that your server is down.

## Firewalls

If you allow our checkers through a firewall, allow our **IPv6** addresses as well as
our IPv4 ones — many firewalls treat the two separately, so an IPv6 check can be
blocked even though you allowed our IPv4. See
[Allowing our checkers through your firewall](/docs/advanced/firewall-allowlist).

## Get started
- [Choose a check region](/docs/configuration/check-regions)
- [Allowing our checkers through your firewall](/docs/advanced/firewall-allowlist)

## Related
- [HTTP / website monitors](/docs/monitors/http)
- [TCP & UDP port monitors](/docs/monitors/tcp-udp)