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Keep Your Virtual Machines and Hosts Talking Without a TCP/IP Connection : Page 2

You don't need to pay for a connection to the Net when you're working remotely. Loopback adapters enable a disconnected machine to have a working TCP/IP stack.

Microsoft Loopback Adapter
The Microsoft Loopback Adapter is software that emulates the operation of a physical network adapter. Click on these links for installing the Microsoft Loopback Adapter in Windows XP and Windows 2000. Instructions on using the Microsoft Loopback Adapter with Virtual Server 2005 are provided here. They are also relevant for Virtual PC.

The Microsoft Loopback Adapter has uses for when a physical network is available as well as when one is not available. When the host is a client of a server (e.g., SharePoint, BizTalk, SQL Server) running in a virtual machine, a loopback adapter enables isolation of the server application from the physical network. You'll particularly need to use this approach when all of the virtual machines are exact copies. When exact copies of virtual machines are started and connected to the same physical network, they have duplicate machine names. This causes a problem because the operating system expects unique machine names on a network (see Figure 1).

Figure 1. Duplicate Name Error Caused By Identical Virtual Machines

You really have only two options for preventing a duplicate name problem. Either give all the machines unique names or isolate the machines from each other so the duplicate machines can't be seen. If you have a lot of machines, both options can be administratively burdensome. Certain server applications such as BizTalk, SharePoint, and SQL Server Reporting Services don't tolerate machine renames very well and require additional reconfiguration after a rename. The Microsoft Loopback Adapter provides a convenient workaround via a TCP/IP network isolated from the physical network. Only the host and the guests can use this network.

Use the Virtual PC console to bind the virtual machines to the Microsoft Loopback Adapter (see Figure 2).

Click to enlarge

Figure 2. Configure Virtual Machine to Use the Microsoft Loopback Adapter

The Microsoft Loopback Adapter creates a virtual network independent of and isolated from whatever physical network(s) to which the host machine is connected. This is good approach in a classroom environment where each student needs to have his or her own application server. The exact same virtual SharePoint server (for example) can be copied to each student's physical machine without any modifications.

If you need to have the host and the guest(s) talk to each other using TCP/IP, you won't be able to using a disconnected network adapter. The workaround is to use a loopback adapter. After adding the Microsoft Loopback Adapter and running IPCONFIG again, notice that the physical adapter is still unusable, but the Microsoft Loopback Adapter has an IP address:


Windows IP Configuration

Ethernet adapter Local Area Connection:

        Media State . . . . . . . . . . . : Media disconnected

Ethernet adapter Microsoft Loopback Adapter:

        Connection-specific DNS Suffix  . :
        IP Address. . . . . . . . . . . . :
        Subnet Mask . . . . . . . . . . . :
        Default Gateway . . . . . . . . . :

Using the Microsoft Loopback Adapter is not always convenient for people who demo often. If the virtual machine does need to access the physical network when it is available, it is necessary to switch the virtual machine back to the physical network adapter.

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