The Rackspace Cloud Authentication Service serves as the entry point to all Rackspace Cloud APIs and is itself a ReSTful web service.
To access the Authentication Service, you must know whether your account is US-based or UK-based:
US-based accounts authenticate through https://identity.api.rackspacecloud.com/v2.0/.
UK-based accounts authenticate through https://lon.identity.api.rackspacecloud.com/v2.0/.
Your account may be based in either the US or the UK; this is not determined by your physical location but by the location of the Rackspace retail site which was used to create your account:
If your account was created via http://www.rackspacecloud.com, it is a US-based account.
If your account was created via http://www.rackspace.co.uk, it is a UK-based account.
If you are unsure how your account was created, use the Rackspace contact information at either site to ask for help.
