To send commands to targeted controllers via the UDP datagrams sent through the Ethernet socket, Internet Protocol (IP) addresses are used. You define programmable controller IP addresses to this device communication enabler by entering host names. Host names are resolved to actual IP addresses by looking them up in the operating system's hosts file. You must ensure that this file contains records for resolving each controller's assigned IP address to a host name. Contact your system administrator or the network manuals for the operating system for instructions on modifying this file. Test the entries by using the ping utility. Ping the controller by the host name in order to insure that each entry is correct and that each controller is reachable.