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IP_ADDRESS   CIDR_SIZE
=========    ==========
10.10         16
15.0          16
67.7          16
18.0           8

WITH ipv AS
(   SELECT  IP.*
        ,   NVL(REGEXP_SUBSTR( ip, 'd+', 1, 1 ),0) * 256 * 256 * 256  -- octet1
        +   NVL(REGEXP_SUBSTR( ip, 'd+', 1, 2 ),0) * 256 * 256        -- octet2
        +   NVL(REGEXP_SUBSTR( ip, 'd+', 1, 3 ),0) * 256              -- octet3
        +   NVL(REGEXP_SUBSTR( ip, 'd+', 1, 4 ),0)  AS ipnum          -- octet4
        ,   32-bits                 AS ignorebits
    FROM  ips IP
)
SELECT  IP1.ip, IP1.bits
FROM    ipv IP1
WHERE   NOT EXISTS
    (   SELECT  1
        FROM    ipv IP2
        WHERE   IP2.bits < IP1.bits
        AND     TRUNC( IP2.ipnum / POWER( 2, IP2.ignorebits ) )
              = TRUNC( IP1.ipnum / POWER( 2, IP2.ignorebits ) )
    )

SQL> desc ips
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 IP                                        NOT NULL VARCHAR2(16)
 BITS                                      NOT NULL NUMBER