Filter an IP with Variable part

Hello everybody!
I am receiving logs from several routers in a building with 20 companies. I would like to identify my top 10 Internet destinations, but I would like to see the Name of the destination in my Kibana graphs, not the IP.
So I guess I might use DNS resolving (but I am afraid it's gonna be too many requests)... or I might use the Filter and Mutate function, but I need some help:

filter { if [dst_ip] == "" { mutate { replace => [ "dst_ip", "BLUE-WEBSITE" ] } } }

Works fine for 1 IP. But BLUE-WEBSITE has more than 1 IP, it has several in the same subnet. So I would like to do something like this, can I?

filter { if [dst_ip] == "" { mutate { replace => [ "dst_ip", "BLUE-WEBSITE" ] } } }

But this doesn't work at all, the XXX is not understood.

Does anyone has any clue? Thank you so much!

Hi there!

I've just found something that seems to work:

filter { if [dst_ip] =~ "(^31.13.95.)" { mutate { replace => [ "dst_ip", "BLUE-WEBSITE" ] } } }

But I am looking forward to read your good suggestions.

Have a look at the translate filter.