Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

Question about IP addressing

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!

2 REPLIES
Cisco Employee

Re: Question about IP addressing

Hi Chris,

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

Regards,

Ankur

Hall of Fame Super Silver

Re: Question about IP addressing

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

114
Views
0
Helpful
2
Replies