Find every subdomain
of a domain.

Every hostname under a domain that ever appeared in a public TLS certificate — with certificate counts and first/last seen dates.

15 hosts · 269 ms

HostCertsFirst seenLast seen
send.springtheory.com 2 2026-06-04 2026-06-04
unsubscribe.springtheory.com 1 2026-10-08 2026-10-08
blog.springtheory.com 6 2026-06-18 2026-09-26
reach.springtheory.com 4 2026-05-07 2026-07-06
research.springtheory.com 10 2026-05-25 2026-08-02
click.springtheory.com 2 2026-10-09 2026-10-09
talk.springtheory.com 4 2026-05-07 2026-07-06
mail.springtheory.com 4 2026-05-07 2026-07-06
preferences.springtheory.com 1 2026-10-08 2026-10-08
classes.springtheory.com 3 2026-06-24 2026-08-25
connect.springtheory.com 4 2026-05-07 2026-07-06
www.springtheory.com 10 2026-04-15 2026-10-09
max.springtheory.com 4 2026-05-07 2026-07-06
mx.springtheory.com 4 2026-05-07 2026-07-06
directory.springtheory.com 2 2026-05-13 2026-10-08

Programmatic access: API GET /v1/hosts/springtheory.com (API key for paging).

Shown here: hostnames with a certificate that is valid now or expired within the last 90 days. The full history since 2013 — hostnames whose last certificate expired earlier — is a Pro plan method: GET /v1/hosts-history/springtheory.com.