A tool that can combine subnets.

I am working on some default address lists for firewalls and wanting to block off huge blocks of the internet.

My problem is that there are allot of subnets and thus allot of entries in the lists.

Since i am blocking everything except 12 to 30 countries and many have almost overlapping subnet domains i am looking for a tool that can unite complementary subnets so that i limit the number of entries in the address lists and gain some performance.

Is this a thing?Or am i on the long road of DIY?