Mpd implementing dynamic IP addresses pool management. When user is authenticated and auth backend have not provided IP address, it can be taken from pool. To make work it you should create one or more address pools with 'set ippool add ...' commands and define which pool should be used with 'set ipcp ranges ... ippool ...' command or RADIUS Framed-Pool attribute.
set ippool add pool first last
This command creates new IP address pool if it not exists and adds specified address range to it.