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Second IP address on single NIC will not work.

Posted on 2008-01-31
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Last Modified: 2013-12-23
I'm trying to make some second IP address work on a network. I've added the IP addresses of 10.42.7.5 and 10.42.7.6 on a couple of machines by using the advanced tab on the TCP/IP properties of the network connection in windows. However when I try to ping from one to the other it doesn't work. I've included the "ipconfig /all" and "ping" results from one of the machines. The wierd thing is that when I try to ping one of these new addresses, I get a reply on the first try followed by three "Request timed out." (please see attached code snippet).
Subsequent ping attempts will result in 100% loss, but if I try again after awhile or after a restart, then I get the 75% loss all over again followed by 100% loss on subsequent tries.
Any help would be appreciated.
Ethernet adapter Local Area Connection:
 
        Connection-specific DNS Suffix  . :
        Description . . . . . . . . . . . : Broadcom NetXtreme 57xx Gigabit Controller
        Physical Address. . . . . . . . . : 00-1D-09-13-58-B9
        Dhcp Enabled. . . . . . . . . . . : No
        IP Address. . . . . . . . . . . . : 10.42.7.5
        Subnet Mask . . . . . . . . . . . : 255.0.0.0
        IP Address. . . . . . . . . . . . : 10.4.3.40
        Subnet Mask . . . . . . . . . . . : 255.0.0.0
        Default Gateway . . . . . . . . . : 10.4.3.1
                                                      10.42.7.1
        DNS Servers . . . . . . . . . . . : 10.4.3.3
 
C:\Documents and Settings\cad>ping 10.42.7.6
 
Pinging 10.42.7.6 with 32 bytes of data:
 
Reply from 10.42.7.6: bytes=32 time<1ms TTL=128
Request timed out.
Request timed out.
Request timed out.
 
Ping statistics for 10.42.7.6:
    Packets: Sent = 4, Received = 1, Lost = 3 (75% loss),
Approximate round trip times in milli-seconds:
    Minimum = 0ms, Maximum = 0ms, Average = 0ms

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Question by:Spelurker
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by:briancassin
ID: 20792238
I am not quite clear on your first statement if you are assigning the same IP address to two different computers you will have an IP address conflict. Even if it is the alternate configuration I would not recommend this.

The alternate configuration only works if DHCP is not available it is a fallback configuration
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by:tgtran
ID: 20792259
Why are you having 2 gateways?
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by:steezy
steezy earned 2000 total points
ID: 20792266
I think you should decide on what you are trying to accomplish and ask that question instead.
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by:Spelurker
ID: 20792370
-They are all static IP's. I'm assigning second IP's to individual machines, not the same IP addresses. (example 10.4.3.40 and 10.42.7.5).
-I threw the second gateway in there just to see if that would resolve the problem; it didn't. 10.4.3.1 is the original working gateway. I'll remove that.
-What I'm trying to accomplish is this: We have to add a network device (It will act somewhat like a specialized gateway that attaches to a PES/satellite) that is in the 10.42.x.x range and the other machines need to be able communicate with it. All of the machines are in the same subnet (10.4.3.x). I was told by the vendor that all I would need to do is add a second IP address to the individual machines in the range of 10.42.x.x.
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steezy earned 2000 total points
ID: 20792449
well if you are using the 10.0.0.0 network with a subnet mask of 255.0.0.0, the IP's those computers already had are in the same subnet as the 10.42.0.0 addresses. In this configuration, anything starting with "10." would be in the same network.
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Author Comment

by:Spelurker
ID: 20854809
Fixed it. The problem was that the VLAN on the Cisco switch needed to have a secondary network added to it (10.42.x.x). Still doesn't explain why the first ping went through, but not any of the subsequent ones; but, alls well that ends well.
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Expert Comment

by:layer9corp
ID: 21460166
We hate to see questions like this left opened. In network engineering the alls well that ends well scenario doesn't really work. Everything happens for a reason, usually user error. In your scenario theres too many variables that you provided no information on.  A prime example is the VLANS, which you failed to mention up front. How those VLANS are configured is anyones guess, so why your ping went through is likewise anyones guess, but its most likely related to a cross connection somewhere on your VLANs. What you want to do in the future is approach these sorts of errors from a pragmatic approach. That means first and foremost, utilizing a "Sniffer" whenever possible. A network engineer is blind without a protocol analyzer, and given that they are free, see "WireShark" (formerly Ethereal) theres no excuse for troubleshooting a network issue without one, particularly if you have a cisco switch capable of Port Spanning, which yours most likely is given it is VLAN capable.  Simply span the associated ports and examine the packet captures. The answers always there.

Also you might try examining the ARP cache next time. This is a simple process of simply typing "arp -a" from a DOS command prompt on the involved system. This will show you whether or not the computers are communicating at Layer 2 (where VLANS operate) which in turn will show you where the problem likely lies. In your case, the two systems being on seperate VLANS means that your ARP cache would have been empty, at least on one machine. This would have alerted you to Layer 2 blocking, hence VLANS.  

Also, one final note, you mentioned you once tried setting the gateway. Obviously you learned that didn't work. Just FYI, you're fine with the secondary addressing for certain applications like NAT applications but you can only have one gateway. If you're troubleshooting this stuff in the future do your IPCONFIG, and then do an arp -a. Lots of answers lie in the arp cache.

Layer9
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