From: Brian Candler <b.candler@...>
Date: 2009-03-02T06:12:02+09:00
Subject: Re: Gets the broadcast address of an IP address

Eleanor McHugh wrote:
> On 28 Feb 2009, at 22:49, Hassan Schroeder wrote:
>>> The simplest way to do it is to use a regex to change the last term  
>>> in the
>>> quad:
>>>
>>> IPSocket.getaddress(Socket.gethostname).gsub!(/.\d{1,3}$/, '.255')
>>
>> Simple but not necessarily correct  :-)
>>
>> <http://learn-networking.com/network-design/how-a-broadcast-address-works 
>> >
> 
> True, but I'm not sure the general case is so easy on the eye ;p

This is messier than it should be, but:

require 'ipaddr'
n = IPAddr.new("192.168.0.101/24")
p n | (~n.instance_variable_get(:@mask_addr) & IPAddr::IN4MASK)

Or maybe you could just broadcast to the all-ones address 
(255.255.255.255)
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