Question about IP addressing

Unanswered Question
May 21st, 2007

This popped into my head and I couldn't figure out the answer:

Say you have a LAN that uses the 20.0.0.0 255.255.0.0 subnet. If Host A on that LAN needs to send a packet to a destination IP 20.0.0.10/16, Host A will ARP for it. What is there happens to be a Host B on the local LAN with IP address 20.0.0.10/8. The ARP request doesn't send out a subnet mask. Won't Host B mistakenly think the ARP request is for it?

Any input is appreciated!

I have this problem too.
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ankbhasi Mon, 05/21/2007 - 23:20

Hi Chris,

Yes the host B will reply to the ARP request and they will be able to communicate with each other.

Regards,

Ankur

Richard Burts Tue, 05/22/2007 - 05:26

Chris

Ankur's answer is correct. But I would like to go a bit deeper into the question and give a slightly different answer. The question suggests that there is a host with 20.0.0.10/16 and also Host B with 20.0.0.10/8. That is a serious problem in the network because now there are 2 hosts with duplicated IP address (the fact that they have different masks does not make them different addresses). If there are 2 hosts with the same IP address, then both hosts will respond to the ARP request and Host A will choose one of the responses and use that MAC address. It is therefore unpredictable which of the hosts will actually receive the IP packets.

It is a cardinal rule of IP networking that there should not be duplicated host addressing within the network. This question violates that principle.

HTH

Rick

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