# apt-get install whois
# host facebook.com
facebook.com has address 157.240.209.35
facebook.com has IPv6 address 2a03:2880:f14e:82:face:b00c:0:25de
facebook.com mail is handled by 10 smtpin.vvv.facebook.com.
# whois 157.240.209.35 | grep CIDR
CIDR: 157.240.0.0/16
# iptables -A OUTPUT -p tcp -d 66.220.144.0/20 -j DROP
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