I'm also facing this issue. Seems like GPIB-ENET/100 is supported in ni-488.2 version 21.0+, but gpibexplorer is failing to find the device. It says the IP/HOSTNAME is not a valid GPIB-ENET/1000 device.
Of course, it isn't... it's an ENET/100!
Did gpibexplorer lose support for enet/100s?
I never got an answer to this - Apparently, they removed the support for the GPIB-ENET/100 in the same release they did the work to deal with the flood of fake USB adapters.
I ended up getting a good deal on a GPIB-ENET/1000 so I just stopped looking.
Sorry, I can't help more.
Did anyone get any resolution with this support of the ENET/100's under Linux? Under the hardware support page on NI, it says these devices should be supported.