Registering a Salesforce Sites Domain
Your company's Salesforce Sites domain is used for all the sites that you create. You must register your company's Salesforce site domain to create public websites and applications that are directly integrated with your Salesforce.com organization—without requiring users to log in with a username and password. Your Salesforce site domain must be unique and must consist of only alphanumeric characters. Salesforce.com recommends using your company's name or a variation of your company's name, such as 'mycompanyportal.'
You cannot modify your Salesforce site domain name after the registration process.