Hello,
The Getting Started with you GPIB-ENET/100 manual has information regarding programmatically setting of the IP address of the GPIB-ENET.
Here is the link to that manual:
http://digital.ni.com/manuals.nsf/websearch/FDBF3832D6A08B838625693C005BF27B?OpenDocument&node=132100_US
Also, why would you like to programmatically change the IP address of your GPIB interface? If you are going to be changing the address a lot, it is a good idea to install a DHCP server somewhere on your network instead of using static IP addresses.
Please let us know if you have further questions.
Anthony Bacak
National Instruments
Applications Engineering