you can (in ver.8) group by
SUBSTR(ip_addr, 1,
LOCATE('.',ip_a ddr,
LOCATE('.',ip_a ddr,LOCATE('.', ip_addr)+1)+1)-1)
becase FROM SYSIBM.SYSDUMMY 1
SELECT SUBSTR('172.18. 196.12',1, ...

You can (in ver.8) group by
SUBSTR(ip_addr, 1,
LOCATE('.',ip_a ddr,
LOCATE('.',ip_a ddr,LOCATE('.', ip_addr)+1)+1)-1)
because
SELECT SUBSTR('172.18. 196.12',1,
LOCATE('.','172 .18.196.12',
...