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Keep Your Virtual Machines and Hosts Talking Without a TCP/IP Connection : Page 3

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.


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Physical Loopback Adapter
A physical loopback adapter is an RJ-45 device that plugs into the host's physical network adapter (see Figure 3). It changes the adapter from a "Media disconnected" state to a functioning state:

C:\>ipconfig Windows IP Configuration Ethernet adapter Local Area Connection: Connection-specific DNS Suffix . : Autoconfiguration IP Address. . . : 169.254.25.129 Subnet Mask . . . . . . . . . . . : 255.255.0.0 Default Gateway . . . . . . . . . :

With a physical loopback adapter plug, the host and the guest(s) can communicate just as well as if the host's network adapter really was connected to a physical network. What is particularly desirable about this approach is that there is no need to reconfigure Virtual PC when switching from a connected to a disconnected state and vice-versa. It is as if the loopback adapter plug becomes a network.



Click to enlarge

Figure 3. Physical Loopback Adapter Plug Inserted Into Laptop Network Adapter

I tried making my own loopback adapter plug from an RJ-45 and a short length of CAT 5 cable, but it didn't work. More importantly, it wasted my time and didn't look professional. I finally located an entire collection of inexpensive adapter plugs (both loopback plugs and crossover adapters) manufactured by Smartronix.

Crossover Cable
Virtual machines are large, ranging from a few gigabytes to tens of gigabytes or more. Copying such large files can be slow. Even if a network is available and two machines are connected to it, other network traffic may prevent the transfer from occurring at maximum speed. USB is too slow for rapid transfers of large files. For the fastest transfer speed when copying files, connect one machine's network adapter to another machine's network adapter. This requires a hub, switch, or crossover cable. A regular Ethernet cable won't work because in the absence of a crossover, the transmission pin on one machine will be connected to the other machine's transmission pin instead of its receiver pin. (There are exceptions. Some network adapters are autosensing and can automatically adjust. See the Wikipedia entry on Ethernet crossover cable for more information.) A direct Ethernet connection between two machines can maintain 85-percent network saturation (Figure 4).

Click to enlarge

Figure 4. High Transfer Speed Using a Crossover Cable

I used to carry a regular Ethernet cable and a crossover cable, but I abandoned this approach in favor of the convenience of using a crossover adapter. It is small and lightweight, and I can use it to convert a regular Ethernet cable into a crossover cable. It's more convenient than carrying two cables.

Loopback Adapters Deliver Convenience
The Microsoft Loopback Adapter and a loopback adapter plug are useful tools when working with virtual machines. Knowing when to use each maximizes the convenience of virtual machine configurations. Crossover cables can reduce your frustration by making large file copies take place quickly.



John Paul Cook is a database and systems architect in Houston, Texas, and is a Microsoft MVP.
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