Single sign-on is a property of access control of multiple related, but independent software systems. With this property a user logs in with a single ID and password to gain access to a connected system or systems without using different usernames or passwords, or in some configurations seamlessly sign on at each system. This is typically accomplished using the Lightweight Directory Access Protocol and stored LDAP databases on servers. A simple version of single sign-on can be achieved over IP networks using cookies but only if the sites share a common DNS parent domain.
A typical example of SSO is a user who logs on to Outlook Web Access by providing credentials on a form. In one of the e-mail messages that the user receives, there is a link to a document that is stored on a SharePoint server. The user clicks the link, and the document opens without an additional request for authentication. This example relies on the use of persistent cookies.