Diameter realms are administrative domains that have a relationship with the user account. For example, a Diameter client can be common to multiple departments with different domain realm names, for example xyz.com and wvu.com. In some cases, realms can represent different geographical regions. Regardless, the realm names are maintained in Diameter nodes (as routes) and are used for routing of diameter messages. In general, realms can be thought of as a string in NAI (Network Access Identifier) that immediately follows the first ‟.” character in the string (for example, jdoe.example.com).
Host names identify a host within a realm. Together, the host and realm names form a Diameter Identity (DI). Every Diameter message must contain origin-host and origin realm AVP.