How can I get the IP address of the system programmatically?

You can use cl_gui_frontend_services to get the system IP address.

DATA ip_addr TYPE c LENGTH 50.
CALL METHOD cl_gui_frontend_services=>get_ip_address
 RECEIVING
   ip_address           = ip_addr
 EXCEPTIONS
   cntl_error           = 1
   error_no_gui         = 2
   not_supported_by_gui = 3
   OTHERS               = 4.

DATA terminal LIKE USR41-TERMINAL.
CALL FUNCTION 'TERMINAL_ID_GET'
EXPORTING
  USERNAME                   = sy-uname
IMPORTING
  TERMINAL                   = terminal.