I was trying to read up on Ubiquiti's support of IPv6 so one Google search took me to a page where 6rd is being set up on their Edge router. http://community.ubnt.com/t5/EdgeMAX-CLI-Basics-Knowledge/EdgeMAX-Enable-IPv6-support-via-CLI/ta-p/472659 Take a look at the very last example frame at the bottom of the page (discussing the ASSIGNEDPREFIXHERE value).