Microsoft KB Archive/139334

= Both Adapters Can Be Reached in Multihomed Windows Computer =

Article ID: 139334

Article Last Modified on 2/20/2007

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APPLIES TO


 * Microsoft Windows 2000 Server
 * Microsoft Windows 2000 Advanced Server
 * Microsoft Windows 2000 Professional Edition
 * Microsoft Windows 2000 Datacenter Server
 * Microsoft Windows NT Workstation 3.5
 * Microsoft Windows NT Workstation 3.51
 * Microsoft Windows NT Workstation 4.0 Developer Edition
 * Microsoft Windows NT Server 3.5
 * Microsoft Windows NT Server 3.51
 * Microsoft Windows NT Server 4.0 Standard Edition

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This article was previously published under Q139334





SUMMARY
On your network with a server running Windows that has with two network adapters installed (also referred to as a multihomed computer), you are able to ping both adapters on the multihomed server from each subnet even though routing is not enabled on the server.



MORE INFORMATION
A Windows system only maintains one routing table for both network adapters. When a network client from either subnet attempts to access the first or second network adapter, the operating system checks the routing table for the network adapter IP address the client is trying to access. Because the IP address exists in the routing table as a local address, the system responds to the network client. The default gateway address of the client must be the address of the adapter in the non-routing, multihomed server that is on the same subnet.

However, the operating system does not route packets between clients of the subnets on the two network adapters unless routing is enabled.

Steps to Reproduce Problem
 Setup a multihomed Windows computer with TCP/IP installed. Verify that both network adapters are addressed to exist on different subnets, Subnet_A and Subnet_B. Verify that the Enable IP Routing option is clear (unchecked) on the system. To find this option:  From the Control Panel choose Networks. From the list of Installed Network Software, choose TCP/IP Protocol. Choose Configure and then click Advanced.</li></ol> </li> Setup a client on Subnet_A connected to the multihomed server and configure the client with a Subnet_A IP address.</li> Set the client Default Gateway address to the IP address of the Subnet_A NIC in the multihomed server.</li> From the client ping the system Subnet_A and Subnet_B NIC addresses.</li></ol>

NOTE: You can ping the Windows system Subnet_A NIC IP address as expected, however, you can also ping the Subnet_B NIC IP address even though IP routing is not enabled.

Additional query words: prodnt mpr

Keywords: kbnetwork KB139334

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