Help he understand, then. There is widely-deployed code that assumes that
the interface ID is 64 and does not work on anything other than 64 bit
prefix lengths.
if you have code which is hard coded to want to be on a network with a
17.5 bit netmask, then put it on networks with 17.5 bit masks. enjoy.
unless it is doing something useful with that restriction, such as
slaac, you're severly restricting the usability of your product. your
choice.
randy