Ah, if the intent is to test the clients/server in an isolated LAN - then, yes, ULAs might work. Though you can as well use the 2001:db8::/32 and later you can

Ah, if the intent is to test the clients/server in an isolated LAN - then, yes, ULAs might work. Though you can as well use the 2001:db8::/32 and later you can also publish your examples readily. My comment was about any testing that involves the boxes that may want IPv4 or IPv6 connectivity to external world, apologies if I misunderstood.