Subdomain
Definition
Any name to the left of a registered domain, separated by a dot, is a subdomain of it. The owner of the parent creates them at will — no extra registration, no fee, no waiting on a registry. Each one carries its own DNS records, so blog.acme.com can sit on one host while acme.com sits on another and neither knows about the other. They nest as deep as you like: staging.eu.acme.com is a subdomain of eu.acme.com, which is a subdomain of acme.com, with the 253-character total and 63-character label limits as the only ceiling. Browsers treat each one as a distinct origin, which is why cookies, CORS rules and certificates all have to name them explicitly. The oldest and dullest subdomain of all is www.
Why It Matters
Subdomains are the cheapest way to keep unrelated things apart while still looking like one organisation. A marketing site, a status page and a downloads area can live with three different providers and one brand. There is a second, more practical reason they matter: because a subdomain is an ordinary name rather than the top of the zone, it may hold a CNAME record. That single fact makes pointing files.acme.com at a host a one-line change, while doing the same at the bare acme.com needs a workaround. When a host documents an easy path and a hard path, the easy one is almost always the subdomain.
How It Works
You add a record at whichever DNS provider is authoritative for the parent domain, putting the subdomain in the host or name field. Some interfaces want just the label, 'docs', and append the rest for you; others want the whole thing. The record type decides the rest: a CNAME hands resolution over to a target hostname, an A record fixes it to an address. Resolvers cache whatever comes back for the length of its TTL, and the parent domain is untouched throughout — creating docs.acme.com cannot break acme.com. A wildcard record on the host '*' answers for every name you have not defined, which is handy for a platform and risky for everyone else.
Real-World Example
Every site published on 99helpers gets one for free, in the form your-name.99helpers.site. Drop in a 12 MB product manual, call it hart-manual, and hart-manual.99helpers.site is live in seconds with a certificate already issued. Nothing was registered and nothing was configured, because 99helpers owns the parent name and created a label underneath it. If you later want files.hart.co.uk to serve the same manual, that is a subdomain of your own name instead, and a single CNAME at your DNS provider does it.
Common Mistakes
- ✕Typing the full hostname into a host field that already appends the domain — the record is saved as docs.acme.com.acme.com and answers nothing
- ✕Expecting a certificate for acme.com to cover app.acme.com — it does not, unless that exact name or a matching wildcard is listed on it
- ✕Leaving a subdomain pointed at a service you have stopped paying for — a dangling record is how someone else ends up publishing under your brand
Related Terms
Domain Name
A domain name is the readable address people type to reach something on the web, such as 99helpers.site. It stands in for a numeric server address that nobody would want to memorise.
Custom Domain
A custom domain is a name you own, pointed at content somebody else hosts, so visitors see files.acme.com instead of the provider's address. You keep the registration; the provider keeps the servers.
CNAME Record
A CNAME record points one name at another name rather than at an address. Hosts ask for them because the addresses behind their target hostname can then change without you touching your zone.
Apex Domain
The apex domain is the bare name with nothing in front of it — acme.com rather than www.acme.com. It is also called the root domain, the naked domain or the zone apex.
URL Slug
The readable, hand-picked part of a web address that names one particular thing — the 'acme-pitch' in acme-pitch.99helpers.site. It is chosen, short and made only of lowercase letters, digits and hyphens.
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