When you load an extension or filter, IIS checks the ISAPI version used to build the DLL. IIS uses this information to determine which functions the extension or filter can use. If you built the extension or filter with a later version of ISAPI than the server supports, IIS will not load the DLL. ISAPI extensions are completely backwards compatible, so all extensions and filters that ran on earlier versions of IIS should run with the most recent release. The following topics provide a summary of the functions offered by the three most recent releases of ISAPI.