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Help with dual NIC config?

Posted on 2013-10-30
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Last Modified: 2013-11-22
Hi,

I was hoping someone could expain this dual nic config that one of our customer's has.  The first nic is public (and appears to have a nat'd ip) while the second is private (with non-nat'd ip).  Everything is currently working, but I'm curious as to 1) why there is no gateway defined for the public IP?  2) why a nat'd IP for the public? and 3) what is the significance of the default route?  Below is the ipconfig snippet (with the ips changed of course)  along with the default route:

Ethernet adapter Public - 1.2.3.4:

   Connection-specific DNS Suffix  . : users.public
   IPv4 Address. . . . . . . . . . . : 1.2.3.4
   Subnet Mask . . . . . . . . . . . : 255.255.254.0
   Default Gateway . . . . . . . . . :

Ethernet adapter Local Area Connection 4:

   Connection-specific DNS Suffix  . :
   IPv4 Address. . . . . . . . . . . : 5.6.7.8
   Subnet Mask . . . . . . . . . . . : 255.255.252.0
   Default Gateway . . . . . . . . . : 5.6.7.89
-----------------------------------------------------
 IP               MASK        GATE          METRIC
0.0.0.0          0.0.0.0     5.6.7.89       1  

Thanks!
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Question by:blinkme323
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by:ButlerTechnology
ButlerTechnology earned 250 total points
ID: 39612765
A system with multiple NICs will tend to only have a single IP address that has a default gateway.  In a simple NAT network, the IP without the gateway address will be the gateway for the clients on the internal network.  The purpose of the default gateway is to allow the system to access outside resources -- specifically the internet.  The route is the catch all.  If there isn't a matching route than the default gateway is used.

Tom
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Dave Baldwin earned 250 total points
ID: 39612824
Yes, that's the way I have mine setup and for the same reasons as Tom listed.
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by:blinkme323
ID: 39613088
Thanks for the follow up.  That's why I can't get my head around this one since the private nic seems to have the gateway.
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Expert Comment

by:Dave Baldwin
ID: 39613126
That would be Ok if somewhere on that network is a router that goes out to the internet.  That would leave the 'Public' IP for traffic to the server instead of local traffic that goes out to the internet.
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by:ButlerTechnology
ID: 39613782
Just for clarification...

Ethernet adapter Local Area Connection 4:

   Connection-specific DNS Suffix  . :
   IPv4 Address. . . . . . . . . . . : 5.6.7.8
   Subnet Mask . . . . . . . . . . . : 255.255.252.0
   Default Gateway . . . . . . . . . : 5.6.7.89

Is this the NIC that is connected to your ISP?

Tom
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Author Comment

by:blinkme323
ID: 39613883
No, that is private connection for internal routing (server to server, content switch, etc).
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Expert Comment

by:ButlerTechnology
ID: 39614003
This might sound silly - Can you unplugged the cable to your ISP to verify that it is the correct NIC in the system?

Tom
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Author Comment

by:blinkme323
ID: 39614220
Unfortunately, I can't disconnect at this time.
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by:masnrock
ID: 39617510
No need to disconnect anything, just trace the cables and make sure that things are going to the right place. I'm assuming that the client's network is working properly in spite of its quirks? I'd be wondering if the NIC with the public IP address is even in use for anything. If this is the server we're talking about, maybe it had previously been used as a router?
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by:masnrock
ID: 39617913
Better yet, is there a private network making use of public IP addresses? Doesn't make sense, but it wouldn't be impossible either.
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