A subdomain refers to the letters on the left side of a web address. 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:
Root domain: bitly
Top-level domain: com
Using a subdomain as your custom domain
You can use a subdomain as your Custom Domain. This is an effective way to use a version of your website's domain as your custom domain. A good example is what the Wall Street Journal does, using a subdomain, on.wsj.com, for short links.
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.
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.