The IPAddr.new().prefix function does not work as intended. This change
removes the TODO comment to use the function because we can't really
replace the current implementation by that built-in feature.
irb(main):001:0> IPAddr.new('ffff:ffff:ffff:ffff::').to_i.to_s(2).count("1")
=> 64
irb(main):002:0> IPAddr.new('ffff:ffff:ffff:ffff::').prefix
=> 128
Change-Id: I07a340e1716f6ab8db718b89bf4535f989df7929
c4c5372851