What is add-on domain?
The meaning of add-on domain: An add-on domain means an additional domain hosted with the main domain on a shared web hosting account. Generally, shared hosting companies associate every client’s account with only one domain name, which is called the main domain. Moreover, they allow clients to host multiple domains that share the same control panel under the same account. So, any extra domain a client adds to his/her hosting account is called an add-on domain.
The files of an add-on domain reside in a directory within the web space assigned to the main domain, though the add-on domain can be accessed as a separate website from visitors. The client has the flexibility to create email accounts that point to the add-on domain, without any external attachment to the main domain.
The downside of add-on domains is that if one has ten (10) domains, they will all have the same IP address. In terms of SEO, if the owner interlinks the websites, search engine algorithms will not give much weight to the links, since they all originate from the same IP address.
Another disadvantage is that all the domains share the same control panel, and this can be quite problematic, if you want a third-party to administer one of the sites.
Moreover, if one of the sites causes a problem that gets the IP address blocked, all the add-on domains and the main domain will also suffer from that.