gistfile1.txt

$ cat bug.rb 
require "socket"

loop do
   s = TCPServer.new("localhost", 5000)
   p s => Socket.unpack_sockaddr_in(s.getsockname)
end

$ ruby bug.rb 
{#<TCPServer:0x100154950>=>[5000, "::1"]}
{#<TCPServer:0x100154748>=>[5000, "fe80::1%lo0"]}
{#<TCPServer:0x100154590>=>[5000, "127.0.0.1"]}
bug.rb:4:in `initialize': Address already in use - bind(2) (Errno::EADDRINUSE)
	from bug.rb:4:in `new'
	from bug.rb:4
	from bug.rb:3:in `loop'
	from bug.rb:3
$ cat /etc/hosts
##
# Host Database
#
# localhost is used to configure the loopback interface
# when the system is booting.  Do not change this entry.
##
127.0.0.1	      localhost
255.255.255.255	broadcasthost
::1             localhost 
fe80::1%lo0	    localhost
$