Generically, an LDAP schema refers to a specific collection of LDAP object classes and attributes. The Sun Java Communications Suite provides schema definition files that are used to update the Sun Java System Directory Server with a definition of schema that are allowed for the following products: Messaging Server, Sun Java System Calendar Server and Sun Java System Instant Messaging. However, the names Schema version 1 and Schema version 2 imply more than just their unique collections of object classes and attributes. Each schema mode implies a logical layout of the application's domains in Directory Information Trees (DITs) . The structure of these DITs differs radically between the two schema modes. Each structure implies something about how the LDAP information is accessed. This article discusses the history of the two modes, the differences in the logical structures associated with them, and the schema attribute differences.