From a linux terminal just run:
$ host facebook.com facebook.com has address 173.252.120.68 facebook.com has IPv6 address 2a03:2880:2130:7f20:face:b00c:0:25de facebook.com mail is handled by 10 msgin.vvv.facebook.com. $ whois 173.252.120.68 | grep CIDR CIDR: 173.252.64.0/18
and that is it.
