How to find IP-Address ranges from list of IP addresses

Question

I have around 20000 IP Addresses. and I am looking to find the network-block-ranges for these IP addresses. The purpose is to provide information to have hole in the firewall so that these addresses can be allowed. I can looking in a.b.c.d/x bits format. There could be several ranges.

Update: I have IP Addresses already in my apache log files, rather than creating new ones.

Answer 1

You can't determine this from the addresses. You need to know the subnet masks.

Answer 2

I believe you're talking about CIDR. 20000 < 2^15. So you need a free A.B.C.D/15 block, but AFAIK /15 block is not common, while /16 is accepted one. So you need A.B.C.D/16.