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Multiple IP Addresses on a Single NIC

This article was previously published under Q149399
A computer with two IP addresses assigned to its interface is unable toconnect to computers across a router.
A single network interface card (NIC) configured with multiple IP addressesdefined on different logical subnets will always use the first IP addressdefined in the TCP/IP configuration as its source address, regardless ofthe destination.
To resolve this problem, upgrade to Windows NT Workstation and Serverversion 4.0.

The problem does not occur if the computer has one NIC per IP address (twoNICs).
Microsoft has confirmed this to be a problem in Windows NT versions 3.5 and3.51. This problem was corrected in Windows NT Workstation or Serverversion 4.0.
Consider the following scenario:

A computer with a single NIC and two IP addresses--the first is,the second on the same segment where two routers arepresent, router 1 with IP address and router 2 with IP address130.1.7.100.

The route table is as follows:

Network Address  Netmask         Gateway Address  Interface   Metric0.0.0.0   1127.0.0.0   1130.0.0.0   1130.0.3.2   1130.0.255.255   1130.1.0.0   1130.1.7.200   1140.1.0.0   1(Previous entry is an added static route)   1255.255.255.255   1				

Note that the interface is always Network is on theother side of router 2 ( and a static route was added for it.With this configuration, connectivity to any host on the local subnet withan IP address of 130.0.x.x or 130.1.x.x is possible. Connectivity to anyhost on the other side of router 1 ( is also possible.Connectivity to anything beyond router 2 is not possible. A network traceshows that the source IP address is when trying to reach adestination over router 2. Router 2 only knows about network anddoesn't have a route defined for network

  • A static route can be added in router 2 to fix this problem (i.e. route add

Article ID: 149399 - Last Review: 11/01/2006 06:09:18 - Revision: 3.1

  • Microsoft Windows NT Workstation 3.5
  • Microsoft Windows NT Workstation 3.51
  • Microsoft Windows NT Server 3.5
  • Microsoft Windows NT Server 3.51
  • KB149399