Open is an overridden procedure in TIdIOHandlerSocket used ensure that a valid Binding exists and has been established for the IOHandler. 

Open calls the inherited Open method. 

When the socket handle for the connection in Binding has not been assigned, Open allocates resources for the Binding property. If Binding has already been assigned in a previous call to Open, the socket handle is reset. 

Reallocating the Binding in Open causes several actions to be performed in order to connect the socket-based IOHandler. 

• OnBeforeBind is triggered to allow custom configuration using the event handler procedure for an associated TIdTCPClientCustom or descendant instance.
• The socket handle and IPVersion values for the Binding are updated.
• Values in BoundIP, BoundPort, BoundPortMin, and BoundPortMax are assigned to the Binding.
• The value in UseNagle is set using the socket options for the socket handle.
• OnAfterBind is triggered to allow processing using the event handler procedure for an associated TIdTCPClientCustom or descendant instance.

Open connects to the remote system using the values in Host and Port when valid values have been assigned to the properties. 

Set the values in Host and Port prior to calling the Open method. 

Set the value in UseNagle prior to calling Open to indicate if the send coalescing algorithm is enabled in socket options for the connection. 

Use Close to disconnect the socket handle for the IOHandler connection.

TIdIOHandler.Host 

TIdIOHandler.Port 

TIdIOHandler.BoundPortMin 

TIdIOHandler.BoundPortMax 

TIdIOHandlerSocket.Binding