Consider the scenario, I have created 2 VM in Azure namely "Master" & "Slave" with static ip's of "10.0.0.5" & "10.0.0.6" respectively. Firewall disable in both the machines.

Master(10.0.0.5) able to ping Slave(10.0.0.6) and vice versa.

Add one additional IP "10.0.0.7" (same subnet) in Master, it works in Master VM.

Its is not resolved by Slave(10.0.0.6) When ever I ping 10.0.0.7 in slave I got this

C:\Users\arun>ping 10.0.0.7
Pinging 10.0.0.7 with 32 bytes of data:
Reply from 10.0.0.6: Destination host unreachable.
Reply from 10.0.0.6: Destination host unreachable.
Reply from 10.0.0.6: Destination host unreachable.
Reply from 10.0.0.6: Destination host unreachable.
Ping statistics for 10.0.0.7:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),

For tracert

C:\Users\arun>tracert 10.0.0.7
Tracing route to 10.0.0.7 over a maximum of 30 hops
  1  Slave [10.0.0.6]  reports: Destination host unreachable.

What am I missing?

  • You can't just give the machine another IP. Follow this guide azure.microsoft.com/en-us/documentation/articles/… – Drifter104 Oct 25 '16 at 11:13
  • Hi, how did you add the 2nd IP Address? – Michael Brown Oct 25 '16 at 17:33
  • ncpa.cpl --> Ethernet --> Properties --> IP4 Properties --> General --> Advanced --> IP Settings --> Add – Arun Oct 27 '16 at 9:21
  • @Drifter104 I have raised the request for MultipleIPsPerNICPreview to Microsoft they have responded as, "We are currently working on a fix and our next update about this feature will be on Monday 10/31/2016 at 5pm US PT." So I have to wait to test this... – Arun Oct 27 '16 at 9:50
  • @arun did you read ther article Drifter104 provided? It clearly shows you the right way to add additional IP's through Azure PowerShell, you cannot just add them through the Windows network control panel item. – Sam Cogan Oct 27 '16 at 20:55
up vote 2 down vote accepted

I encountered same issue before. Then I tried to perform a network capture on the Azure VM and found something interesting.

The VNET doesn't work like the physical Ethernet device. All communication between Azure VM is handled by gateway.(Including VMs in same subnet, which is different with the physical network.)

ARP request will not be received by the destination. Gateway will intercept it and reply it with a fake MAC address 12:34:56:78:9a:bc. Then the sender will send the packet to gateway and the gateway will forward it for the sender.

If we add an additional IP address on the VM which is unknown to Azure, guess what will happen? Bingo, the gateway will not response to an ARP request containing a unknown destination. So that the sender's ARP request will fail and the error message "Destination host unreachable" shows up.

Here is the screenshot of my lab:

enter image description here

Hope my experience helps.

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