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I've been given access to a Windows Server 2008 R2 Enterprise machine that was provisioned like this:

  • network 1.2.3.136/30
  • gateway 1.2.3.137
  • IP 1.2.3.138
  • broadcast 1.2.3.139
  • netmask 255.255.255.252

My understanding is that anything sent to 1.2.3.139 (the broadcast address) is also sent to 1.2.3.138. Is that correct? I'd rather treat .139 as a single dedicated address so I can run a program receiving on .138 and another program receiving on .139 and not have .138 see things received by .139. Is that possible?

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No. The Broadcast IP that you have noted emerges based on the IP network characteristics (the IP and the Subnet Mask), and is always the highest host address in the network the IP/Mask define.

I say that it "emerges" because the Broadcast address is not configurable. Every computer that joins that network needs to intuitively predict its Broadcast address from the Address and Mask provided, so the IP protocol defines how any host can calculate what Broadcast address it should use. You couldn't change the broadcast on any one system, every system on the network has the need to calculate the broadcast address (which is part of why you can't configure it).

As such, the only person who could change the broadcast IP, are the people that set the network Identity and Mask, and they are constrained by the IP protocol, so it will always be the highest address in the range. Any /30 network starting at 1.2.3.136 will have its Broadcast at .139.

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