A subdomain refers to variations on the left of the domain, while the middle part is called the root domain and the characters at the end are the top-level domain.
As an example, let's look at the URL for the Bitly Help Center, support.bitly.com:
Subdomain: support
Root domain: bitly
Top-level domain: com
You can use a subdomain as your Custom Domain. Be sure to configure your DNS record accordingly.
One exception to using a subdomain as your custom domain is www. While www.your-url.com is technically a subdomain of your-url.com, there is a general convention on the web that it should have similar (if not the same) content. Accordingly, you can only use www.your-url.com if your-url.com is configured as the custom domain.
Keep these things in mind as you choose and set up a custom domain:
- The domain for your short links must be different than your website domain.
- It can't be more than 32 characters long, including the dot.
- If you don't have one yet, Bitly offers a complimentary domain on some paid plans.