Who is invading my computer?

I have a similar icon:

So, what exactly are those funny computers that show up in the Finder window sidebar? A good place to start might be to find out how those icons get in your sidebar.

Just like almost every application running on your computer , the Finder has a Preferences dialog box:

Opening the Preferences dialog box shows a window with several different options in the toolbar at the top of the window. Today we are going to focus on Sidebar.

Those checkboxes control exactly what you will see in the sidebar. If you remove the checks from all the boxes, the icons and even the word “Shared” will disappear from the sidebar.

So, let’s see what appears as we re-check the boxes.

Because of the router provided by my ISP (Internet Service Provider), Back To My Mac will not work, so we will begin with both Back to My Mac and Connected Servers checked.

Now we can see my iMac and another icon, All…

We are seeing my iMac because I have File Sharing turned on. If I turn File sharing off, then that icon would disappear. I have several external FireWire drives connected to my iMac. I use them as a place to store extra files so that I can access them from my other computers, so I need to use File Sharing.

Let’s consider that All… icon. Mac OS X makes it very easy to set up a home network. So, Apple wants to make sure you can get to your other computers. That All… icon makes sense. It is a link to the other computers on your personal network..

This time, I have added a check to Bonjour Computers and my icon with the funny name appears again. Why does this happen?

Let’s think of how computers advertise themselves. Every device that can access the Internet is assigned a unique MAC (Media Access Control) address as it is manufactured. In the case of your Macintosh computer, there is a MAC address assigned to the computer itself. That address is used to identify your computer when it is connected to the Internet or to a computer network by Ethernet, by Bluetooth or by FireWire. However, when your computer is connected to the Internet or a network using its AirPort card (WiFi), it displays the MAC address of that card. So, if your computer has a wireless card, there will be two MAC addresses associated with it. If you have a Mac Pro with an extra Ethernet card in a PCI slot, then you would have a third MAC address for your computer. If you have a MacBook Pro and you have a wireless card in the Express Card slot, then you will have an additional MAC address associated with your computer.

Since I do not have a Mac Pro with an extra Ethernet card and I do not have an additional wireless card in the Express Card slot of my MacBook Pro, I cannot show you how to access those MAC addresses. But I can show you how to find out the MAC addresses of your computer and your AirPort card.

First, let’s find your AirPort Mac Address. Begin by opening your System Preferences, either from the icon in your dock or by selecting it from the Apple menu:

Now locate the Network icon in the System Preferences window. Click it.

In the Network window, select AirPort in the left column, then click the Advanced… button in the lower right corner of the window.

Select the AirPort tab in the window that appears and you will see your AirPort ID (MAC address) near the bottom of the window. You may want to write the ID down or copy and paste it into a place where you save important information about your computer.

Now, click the Cancel button. You will then be back at the main Network screen.

Now let’s figure out the MAC address of your computer when it is not using AirPort.

You are in the main Network screen. This time select Ethernet and click the Advanced button in the lower right corner of the window.

You will need to choose the Ethernet button in the next window.

You will see the Ethernet ID (MAC address) under the row of buttons. Once again, you may want to copy and paste this number into a place where you store important information about your computer.

You should also find the MAC addresses for the other computers that you own and put this information into a document that contains the important information about your computers.

Let’s go back to that screenshot of my computer network. We can see that the strange icon’s MAC address ends in a6. Looking at the document above, we now know its the MAC address of the Ethernet port on my iMac. It is not in use right now, but I used it when I did the first backup to my Time Capsule. It remains from that time.

Now, let’s look at Carl’s screen shot. I know that Carl uses Parallels to run Windows on his computer. I suspect that he has file sharing turned on in the Windows environment and that this icon represents File Sharing or network activity in Windows.

By the way, did you know that your wireless devices such as AirPorts, routers, modems, Time Capsules, Apple TVs and even networked printers also have MAC addresses!

Who says we don’t live in a MACworld!

If you have problems on your network or if you would like to have the experts at Bob LeVitus Consulting assess your network security, give us a call at 408 627-7577 or send an email to urgentrequest@boblevitus.com. We will give you a quote for the cost of an assessment.

--Pat