DefaultPort is a TIdPort property that identifies the default port number used to listen for new client connections to the server. 

Reading or writing the value in DefaultPort causes the corresponding value in the Bindings collection to be accessed or updated. DefaultPort is the port number to use when a port number has not been assigned for a TIdSocketHandle instance in the Bindings collection. The port number is used when the socket handle for the TIdSocketHandle instance is allocated and bound during startup for the server. 

Change the value in DefaultPort prior to creating Bindings or setting Active to True to startup the server. Changes to the property value are ignored after Bindings are bound and the server becomes Active.

Active 

Bindings 

TIdPort 

TIdSocketHandle 

TIdSocketHandles