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Delphi LAN Problem.

Posted on 2008-06-11
Last Modified: 2013-11-23
Hello, We have 1 PC with 3 ethernet cards install connected to 3 different networks.

I got an IP address some where on one of my 3 networks, How can I detect using what ethernet card Windows will use to rich the IP address more accurate I want to know the local IP address of the ethernet card on which the remote IP is located.
Question by:S_Warrior

Expert Comment

ID: 21769048
You wouldn't actually use Delphi to try to figure out what what network card traffic will go from to try to reach a specific IP address, you'd look at your routing table and ascertain that from there.  Delphi does not choose a network interface, that's all based on your winsock layer, but you can choose which interface IP to bind to, if you want it to go through one specific interface.  Bind to the IP address of the interface you want to use, if this is the case.

To view your routing table, open up a DOS box, and then type ROUTE PRINT.  See if your target network is listed, and if it is, it will tell you what the interface IP would be of the outbound route.  If it is not listed, then it will use the default gateway to reach that network host.  I suspect that you will probably find that it's going through the interface which has the default gateway set.

Author Comment

ID: 21769787
No, No, My application must detect this automatically.
LVL 18

Expert Comment

ID: 21769884
What are the IP addresses of the three network cards inside your PC?

That would be a first big step in helping!

Are all three IPs static or are they dynamic?

Same subnet?

Same Net? (Bad idea).

Some more information is truly needed here I believe.


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Author Comment

ID: 21776635
I think u do not understand! I am not trying to find it for my pc I am trying to write an application that will detect it.

LVL 37

Expert Comment

by:Geert Gruwez
ID: 21863318
open a dos box with cmd
and type
ipconfig /all

Author Comment

ID: 21881352
Geert_Gruwez:this is the most stupid's answer I ever got on EE. read the questions.

Accepted Solution

alkisg earned 250 total points
ID: 21889318
The function you're looking for is GetBestInterface:

Then you may query the returned interface for its IP, netmask or whatever else you like.

Expert Comment

ID: 22231033
If IP address you try to access from the PC is in the same subnet with one of your interfaces then windows will use that interface to reach the IP address.
If IP address you try to access from your PC is in different subnet windows will use first adapter in binding order that have default gateway configured.

Expert Comment

ID: 22293331
Hello S_Warrior,

bjove is absolutley right. But I thought you umight need some further information. To check wether an ip address resides in a local area network you have to compare the host part of the ip address provided with the host part of the ip address of one network interface, like (ip_provided and subnetMask_interface) = (ip_interface and subnetMask_interface). You can get the ip addresses and subnet masks of all interfaces using the IP Helper API function GetAdaptersInfo (http://msdn.microsoft.com/en-us/library/aa365917(VS.85).aspx). This adapters info table also includes the configured gateways.


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