You can explore related site names by slightly changing an existing site name. For example, if you find a site name has a fourth-level or higher domain name, you can try removing the left-most parts to see if there are related sites, as in:

www.main.twenty.net —> www.twenty.net

If a site name includes a number, you can try related numbers. For example, you can change

www3.twenty.net —> www2.twenty.net

Similarly, you can change:

www.twenty.net/news/story1.html —>
www.twenty.net/news/story2.html

You can also change the site suffixes to explore related sites. Sometimes there are related sites, sometimes they are owned by the same company, and sometimes there isn’t anything there. For example, you can try each of the following sites:

www.time.com
www.time.org
www.time.net

You can also try different prefixes, such as “ftp” and “support” in place of “www” to find sub-sites.