group ip address to different subnet with shell

Actually, this is for a redis cluster, I have 3 master ips and 3 slave ips, need to produce 3 pairs of IP addresses where the addresses in the pair are in different /24 subnets. Or in other words, given a list of n IP addresses (n being an even number, and not more than half of the IP addresses being in the same /24 subnet), how to partition them into n/2 pairs where the two addresses in each pair are in different /24 subnets? The pairs should be stored as key and values of an ip_map associative array. For example, given a list of IP addresses stored in a $ips array: ips=( "172.211.91.63" "172.211.90.61" "172.211.91.30" "172.211.90.173" "172.211.89.233" "172.211.89.166" ) The result could be: declare -A ip_map=( [172.211.91.63]=172.211.90.61 [172.211.91.30]=172.211.89.233 [172.211.90.173]=172.211.89.166 )