Yes — substack.com has IPv6.

Has both IPv4 (A) and IPv6 (AAAA) addresses.

This is a saved measurement from 2026-10-31 12:10 UTC. Check substack.com live now

B
IPv6 readiness
Works over IPv6, with one gap: its mail servers lack IPv6.
Website over IPv6
substack.com has 2 IPv6 addresses and answers on every one we tested.
✓ 2606:4700:4404::6812:25c8 answers · 443✓ 2606:4700:4401::ac40:9638 answers · 443
yes
www over IPv6
www.substack.com has 2 IPv6 addresses and answers on every one we tested.
✓ 2606:4700:4404::6812:25c8 answers · 443✓ 2606:4700:4401::ac40:9638 answers · 443
yes
Mail servers (MX) have IPv6
None of the mail servers has an AAAA record.
mxa.mailgun.org ✗mxb.mailgun.org ✗
no
Nameservers have IPv6
2 of 2 nameservers checked have an AAAA record, so IPv6-only resolvers can look the domain up.
ali.ns.cloudflare.com ✓lee.ns.cloudflare.com ✓
yes
HTTPS over IPv6 · not part of the grade yet
Loads over IPv6 with a valid certificate: HTTP 200.
https://substack.com/ via 2606:4700:4404::6812:25c8 → 200
works

What's keeping substack.com from an A

  1. Mail servers without IPv6
    mxa.mailgun.org, mxb.mailgun.org have no AAAA record, so mail from IPv6-only servers can't reach them directly. If a provider runs your mail, that's their setting; if you run it, add AAAA records for these hosts.

Check again

substack.com  Has IPv6

A (IPv4)172.64.150.56
104.18.37.200
AAAA (IPv6)2606:4700:4404::6812:25c8
2606:4700:4401::ac40:9638

www.substack.com  Has IPv6

A (IPv4)104.18.37.200
172.64.150.56
AAAA (IPv6)2606:4700:4404::6812:25c8
2606:4700:4401::ac40:9638

Browsers on dual-stack networks will usually pick IPv6 here (the "Happy Eyeballs" algorithm races both and prefers v6).

Copy the evidence — a citation-ready record of this measurement

 Permanent link: aaaa.bot/r/f916ROqyhp · How we measure

Checked 2026-10-31 12:10:17 UTC. Full DNS report: dns.pizza/domain/substack.com · JSON: /api/v1/substack.com