What is a subdomain, and do I need one?

The shape of the thing

A domain is example.com. A subdomain puts a word in front of it, separated by a dot: blog.example.com, shop.example.com, members.example.com.

Each one can point somewhere different. That is the whole reason they exist.

The free one you were given

Every account has a systeme.io subdomain. It lives under your profile picture, at Settings, then Custom domains, where the three dots offer Edit subdomain name.

Sub-accounts get one in the same shape. The help centre's example is project-1.systeme.io.

Connecting a subdomain of a domain you own

It starts in the same place as a full domain: Settings, then Custom Domain, then Add a domain. You type the whole thing, subdomain.domain.com, and save.

Two CNAME records then appear, and both go into your host's DNS zone. The first has the subdomain itself as its host name. The second has a long validation string with the subdomain on the end of it, and points at an acm-validations.aws address.

Records take 24 to 48 hours to spread. systeme.io points at dnschecker.org for confirming they landed, searched by the full record name including your domain.

Where it differs from connecting a whole domain

The whole domain guide asks you to enter the name starting with www, and then adds a third step: a redirection so the bare domain reaches the www address. The subdomain guide stops after the two CNAME records.

systeme.io keeps these as two separate articles and each one points at the other, so following the wrong one is a common way to lose an afternoon.

The limit that applies to both

A subdomain can only be used for one site. If it is currently serving an external site, that site stops working at that address once the subdomain is added to systeme.io.

Do I need one?

Not to publish. systeme.io describes the one you were given as the default address, replaced by a custom domain when you add one.

The case for connecting a subdomain of your own is narrower than it looks. Pointing a whole domain at systeme.io takes the entire address, and anything already hosted there stops being reachable. A subdomain takes only that one name, which leaves whatever sits on the main address alone.