--- Resolv.getname(address)
--- Resolv.getnames(address)
--- Resolv.each_name(address) {|name| ...}
    lookups hostnames of address which represents IP address as a string.

    getname returns first entry of lookupped names.
    getnames returns lookupped names.
    each_names iterates over lookupped names.

    :
      Resolv.getname("210.251.121.214").to_s #=> "helium.ruby-lang.org"

