Get ip address from domain name or get domain name from ip in ruby

resolv class can let us get ip from domain name or get domain name from ip in a ruby program.

example

require 'resolv'
p Resolv.getaddress "www.ruby-lang.org"
p Resolv.getname "210.251.121.214"

result

"54.235.116.125"
"ci.ruby-lang.org"

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