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Editorial Team
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Asked: 2026-05-24T02:02:30+00:00

I am using the Microsoft code here to learn how to detect IP addresses

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I am using the Microsoft code here to learn how to detect IP addresses of cards and devices:

http://msdn.microsoft.com/en-us/library/aa365949%28v=VS.85%29.aspx

I notice some strange behavior.

  1. I have a system with two ethernet cards; one is connected to the internet and one is connected to an ethernet device. When I run the sample code, it will always give an IP address for the card that has the internet connection, but the other card will come up as 127.0.0.1 with a subnet mask of 255.0.0.0 unless I have the ethernet device plugged in and powered. But the card should have a default IP address whether its actually connected to anything, right? How can I modify this code to detect that?

  2. There’s a third IP address detected that appears to be just empty data. I tried this on another computer with a single network connection and it also detected a second, non-existent connection. Each time, this connection has an IP address of 127.0.0.1 and a subnet mask of 255.0.0.0. What does this represent?

  3. Given the demo code, would this be easy to edit to be able to detect IP addresses of devices on the network that any card is connected to? I really just want to detect the IP address that a single ethernet device is set to. The device is directly connected to the card. The reason I want to do this is because the device and card obviously don’t play nice when their subnets are different and I want to detect when this is the case.

Thanks!

R

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1 Answer

  1. Editorial Team

    That address of 127.0.0.1 is not the address of the other card. It is the address of the loopback adapter – a virtual IP address that can only send and receive data with itself. The other NIC (that isn’t plugged into a network) is simply not in the address table.

    You may just want to call GetAdapterAddresses and filter out all adapters with an IFType of IF_TYPE_SOFTWARE_LOOPBACK.

    If you want to see use existing tools that provide the same thing, type either of the following from a command prompt:

    route print
(This will dump the routing table)

ipconfig /all
(this will show you the state of ALL adapters including the loopback)
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