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Networking, subnets, and Windows NIC's

Posted on 2012-09-20
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Last Modified: 2012-09-21
Using the following subnets:

Subnet 1

   Network:   10.14.101.104/29    
   Hosts:     10.14.101.105 - 10.14.101.110  
   Broadcast: 10.14.101.111 

   I assigned the following nodes to this subnet

      10.14.101.105  Windows
      10.14.101.108  Linux

Subnet 2

   Network:   10.14.101.112/29   
   Hosts:     10.14.101.113 - 10.14.101.118        
   Broadcast: 10.14.101.119  

   I assigned the following nodes to this subnet

      10.14.101.113  Windows
      10.14.101.114  Linux

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@ of the IP's are actually 2 NIC's on the same Machine (in 2 different subnets), and they send to their respective servers in their subnets

      Windows NIC_1 10.14.101.105  --> Linux Server_1  10.14.101.108
      Windows NIC_2 10.14.101.113  --> Linux Server_2  10.14.101.114

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I was surprised by what I observed.  Traffic only went out of the NIC1, no traffic exited the NIC2.

I thought by using the 2 NIC's in different subnets would move the traffic into both NIC's

Why did this not work ?
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Question by:Los Angeles1
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9 Comments
 
LVL 12

Expert Comment

by:mlongoh
ID: 38419422
What's the subnet mask look like for these machines?
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Expert Comment

by:Fred Marshall
ID: 38419438
Well, you can't have more than one "internet gateway" on the machine.
So one NIC will have a gateway entered in its settings.  Call it NIC 1 on subnet 1.
And, the other NIC won't.  Call it NIC 2 on subnet 2.

Any packet destined for subnet 2 will go to NIC 2.

Any other packet (i.e. NOT destined for subnet 2) will go to NIC 1.
Any packet on NIC 1 which is not destined for either subnet 1 (and, by definition, subnet 2) will be destined to the gateway address.
Any packet on either NIC 1 or 2 that's destined for the local subnet will just go out on the wire.

In your example, I will assume that NIC 1 points to the internet gateway.
Packets destined for subnet 1 will go to NIC 1.
Packets destined for subnet 2 will go to NIC 2.

Try this:
Run a command line:
route print
From this you should see the routing table for both NICs.
And, you should see the default route to 0.0.0.0 going to the NIC with the gateway.
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Author Comment

by:Los Angeles1
ID: 38419630
To ansser an ealier question, both subnets use 255.255.255.248   /29

Also I defined a gateway of 10.14.10.10 on BOTH NIC's.

Route print provides the following
IPv4 Route Table
===========================================================================
Active Routes:
Network Destination        Netmask          Gateway       Interface  Metric
          0.0.0.0          0.0.0.0      10.14.10.10    10.14.101.105    266
          0.0.0.0          0.0.0.0      10.14.10.10    10.14.101.113    266
    10.14.101.104  255.255.255.248         On-link     10.14.101.105    266
    10.14.101.105  255.255.255.255         On-link     10.14.101.105    266
    10.14.101.111  255.255.255.255         On-link     10.14.101.105    266
    10.14.101.112  255.255.255.248         On-link     10.14.101.113    266
    10.14.101.113  255.255.255.255         On-link     10.14.101.113    266
    10.14.101.119  255.255.255.255         On-link     10.14.101.113    266
        127.0.0.0        255.0.0.0         On-link         127.0.0.1    306
        127.0.0.1  255.255.255.255         On-link         127.0.0.1    306
  127.255.255.255  255.255.255.255         On-link         127.0.0.1    306
        224.0.0.0        240.0.0.0         On-link         127.0.0.1    306
        224.0.0.0        240.0.0.0         On-link     10.14.101.105    266
        224.0.0.0        240.0.0.0         On-link     10.14.101.113    266
  255.255.255.255  255.255.255.255         On-link         127.0.0.1    306
  255.255.255.255  255.255.255.255         On-link     10.14.101.105    266
  255.255.255.255  255.255.255.255         On-link     10.14.101.113    266

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Oddly enough, I do not see an entry for either of my servers .108 or .114

Should I take the gateways out of the NIC configurations

Should I modify the route table ?
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LVL 12

Expert Comment

by:mlongoh
ID: 38419639
Only ONE NIC should have a gateway defined.
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Assisted Solution

by:mlongoh
mlongoh earned 1000 total points
ID: 38419650
Clear the gatway value for one of the NICs.
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Author Comment

by:Los Angeles1
ID: 38419725
OK, this worked

I have 2 NIC's, with IP's in different subnets, but only one of the NIC's has a gate way

Last question:  why did this work

Does  the gateway on on NIC and not on another cause Windows to route differently ?
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LVL 26

Expert Comment

by:Fred Marshall
ID: 38419889
I'm amazed that it works because the gateway address isn't on either of the subnets.  
Normally the gateway is in the subnet of the NIC of that same subnet.

Is there a routed path somehow to 10.14.10.10?

Here's a guess:
The subnet mask at 10.14.10.10 is 255.255.0.0.
This will sort of work because the gateway is looking for packets coming to it on the wire.
The NIC apparently doesn't drop those packets so they do go out.
When they hit the gateway, it acts as if 10.14.101.xxx is on its subnet because it *is*.
So returning packets are sent out onto the wire from there.
The only problem are the broadcast addresses.
The broadcast addresses for 10.14.101.104 and 10.14.101.112 are legitimate host addresses in 10.14.0.0 /255.255.0.0 and won't be seen as broadcast at the gateway, etc.

See the attached.
2011-08-26-How-Subnets-Work-in-P.pdf
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Author Comment

by:Los Angeles1
ID: 38420050
Yes, 10.14.10.10 has a mask of 255.255.0.0

I am still confused as to why having 1 NIC with a gateway and another NIC without a gateway worked

Does this mean that the NIC without the gateway simply sent packets on the subnet, and that the NIC with the gateway sent packets  via the gateway

IF so, why, since the 2 NICs are on 2 different subnets
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Accepted Solution

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Fred Marshall earned 1000 total points
ID: 38420074
Does this mean that the NIC without the gateway simply sent packets on the subnet, and that the NIC with the gateway sent packets  via the gateway

In a manner of speaking, yes.  I tried to say that earlier.

If a NIC does not have a gateway address entered then that NIC is not a routing destination for packets that are not destined for any directly connected subnet.  Only packets for its subnet get routed there.

If a NIC does have a gateway address entered then that NIC is *the* routing destination for packets destined for its subnet AND packets that are not destined for any directly connected subnet.

So, the routing table should show this.

But it's not really proper to connect subnets by only using a different subnet mask at the gateway LAN interface as it appears has been done here.  I mentioned the broadcast address issue if this is done.
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