Methods, systems, apparatuses, and computer program products are provided for automatically determining a home realm. An authentication request receiver interface may receive a request to access a resource and a device identifier from a client device. An authenticator may be enacted in response to receiving the request to access the resource that includes a home realm discoverer and an authentication user interface (UI) provider. The home realm discoverer may determine, based at least on the device identifier, the home realm from a plurality of realms. The authentication UI provider may provide, to the client device, an authentication UI via which a flat-name username can be submitted. Based at least on a flat-name user name and the determined home realm, access to the resource may be granted. In this manner, a user may input a flat-name username during sign-in, rather than inputting a realm or an entire e-mail address.