What is a subdomain? July 25, 2025 15:55 Updated Subdomains are used to refer a web address that works as an add-on (or related site) from the main domain name. For instance: for www.register.domains, the subdomain is “www”, and for mail.register.domains, the subdomain is “mail”. Related articles How do I create an A, MX or CNAME record? What is your domain discount policy? What are DNS records?