Many thanks for your fast answer and your support!
Read the IPPool example.
Can you please point me which one you meant? I found http://wiki.freeradius.org/modules/Rlm_ippool, it this the right one?
The ippool module manages IPs in pools. It can deal with multiple pools simultaneously. Just set the Pool-Name attribute.
I think I had a 'hanger' anywhere. From the URL above, in "Custom configuration" - there are 2 ippools defined with different ip ranges (this is the scheme I need). But I did not find out what single name of ippool module I should write into accounting {} and post-auth{}. In the end I gave up this way but succeeded in ip pooling at last by switching to sqlippool where the name of the ippool module is simply "sqlippool" and the pools are handled by radippool. After changing the update clauses from " expiry_time IS NULL" to " expiry_time = NULL" in ippool.conf mysql (with innodb on radippool) does not print errors anymore. The assignment of ip addresses from ip pools dependent on realms works as expected. Thanks again, Uwe Freier