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I have created a Windows loop back adapter with 'Obtain IP address Automatically' enabled.

ipconfig /all

Ethernet adapter LL Address:

   Connection-specific DNS Suffix  . :
   Description . . . . . . . . . . . : Microsoft Loopback Adapter #3
   Physical Address. . . . . . . . . : XX-XX-XX-XX-XX-XX
   DHCP Enabled. . . . . . . . . . . : Yes
   Autoconfiguration Enabled . . . . : Yes
   Autoconfiguration IPv4 Address. . : 169.254.8.151(Preferred)
   Subnet Mask . . . . . . . . . . . : 255.255.0.0
   Default Gateway . . . . . . . . . :
   NetBIOS over Tcpip. . . . . . . . : Enabled

So I believe the adapter has taken a Link Local address. Now we have an embedded networking device in which I hard code the same Link Local address to test AUTO IP Link Local address functionality. This embedded code is developed using VC - so we can compile and run on same host where I have setup the Loopback adapter.

After bringing on the embedded networking code I see ARP request going out stating 'Who has 169.254.8.151' but I see no reply back from Loopback adapter that the IP is at XX-XX-XX-XX Hardware?

Should not the Loopback adapter reply back stating the same? What wrong or extra needs to be done to make this Loopback adapter reply back stating the same?

  • Is the application using the loopback adapter, or only the physical adapter? Which adapter's traffic are you monitoring? – Harry Johnston Jun 30 '18 at 5:31
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Should not the Loopback adapter reply back stating the same?

No. The loopback adapter is a software construct. It is entirely internal to the host.

  • But both of them are running on same host - is there a way I can put both on same logical network? – Programmer Jun 29 '18 at 16:02
  • Updated the question that both embedded software and loopback adapter are on same hosts – Programmer Jun 29 '18 at 16:10
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If you want to reach a loopback address over a network, then you need to have another IP address configured on a real interface, which is operational. In addition to this you need to advertise your loopback address into the routing domain (eg. ospf or bgp). The packets will be routed through your real interface and land on your loopback interface's virtual nic.

  • Now I see 2 ARP probes - one to certify that the another IP and Link Local IP advertising who owns it and not the address conflict. Please explain "loopback address into the routing domain (eg. ospf or bgp). " - what needs to be configured in the loopback adapter? – Programmer Jun 29 '18 at 17:52

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