Ask the Wi-Fi Guru, Episode XXIV

By Aaron Weiss

February 05, 2010

Our monthly Q&A series offers advice to those seeking help with home or small business WLANs. This month our guru helps with a DD-WRT troubleshoot, describes how to divvy up a WLAN and explains how to share a hotel's Wi-Fi connection.

Our monthly Q&A series offers advice to those seeking help with home or small business WLANs. This month our guru helps with a DD-WRT troubleshoot, describes how to divvy up a WLAN and explains how to share a hotel's Wi-Fi connection.


Strictly speaking, this is a column about wireless networking. But if you're in the mood for an exciting peek into the future, check out this video from the 2010 Consumer Electronics Show. In this exhibit, Chinese manufacturer Haier demonstrates a "completely" wireless HDTV. The video signal comes from a wireless DVD player, which is neat, but the bigger "wow" is that its power source is also wireless.

That's right—no power cord! (No, the secret isn't in hidden batteries, but wireless power using technology by Witricity.) And here I thought that Haier only made dorm-sized fridges and microwave ovens. Come to think of it, a wireless fridge and microwave would make for an awesome treehouse.

Sadly, the CES exhibit is just a demonstration—no retail product yet available for us mouth-breathing consumers. Until then, we'll have to stay excited about wireless networking.

Why Isn't My Linksys Router with DD-WRT Working?

Q: I purchased a Linksys WRT54GL as a secondary router and flashed it with v24 of DD-WRT successfully with the intention of setting up a wireless bridge. I was able to join my wireless network and see its signal strength within the DD-WRT control panel. However, after completing all of the settings on my secondary router, I unplugged the network cable from my computer for 10 seconds, then plugged it back in to test whether I could connect to the internet. I could not and the Local Area Connection Status window confirms that I have no connectivity because the network didn't assign a network address to the computer (it is set up to obtain an IP address automatically). I've triple-checked all of my settings and at this point, I'm at a loss. Do you have any suggestions for how I can proceed and get my wireless bridge working? – Melissa

A: The good news, Melissa, is that you've done a great job of troubleshooting the problem. The bad news, of course, is that the problem still exists. Now what? Let's first take a step back.

In a wireless bridge setup, the secondary router (the WRT54GL in this case) will receive the wireless signal from the primary router and share that with wired clients connected to the secondary router.

By default, routers are designed to provide both DHCP and firewall services, but in a wireless bridge configuration the secondary router should not do either of these. Normally, the client connected to the secondary router will be assigned an IP address by the primary router. But in Melissa's case, this is not happening.

What could be wrong? First let's be sure that the firewall is disabled on the secondary router (in DD-WRT, Security/Firewall/Disable SPI Firewall). In a wireless bridge configuration, the secondary router's IP address is manually assigned to be compatible with the primary router LAN. For example, if the primary router is 192.168.1.1, the secondary router could be assigned 192.168.1.2, with a subnet mask of 255.255.255.0 and a gateway of 192.168.1.1. If these settings are not in place on the secondary router, clients may not receive an IP address from the primary router.

Assuming the secondary router is configured to a compatible LAN address, I would next check any security settings on the primary router. If the primary router is using WEP or WPA encryption, these settings must be replicated in the wireless security configuration of the secondary router. If the primary router has a MAC filter in place, be sure that the MAC address of the secondary router's wireless adapter is included in this MAC filter.

If none of these efforts lead anywhere, try configuring the secondary router as a wireless client rather than a wireless bridge. In wireless client mode, the secondary router behaves as a "full-blooded" router­—with DHCP server and firewall intact. Rather than manually assign a LAN IP address, the secondary router will attempt to request an address from the primary router. In DD-WRT, you can look at the upper right corner of any administration page to see the "WAN Address". If this displays 0.0.0.0, then the secondary router has not received an IP address from the primary router, meaning that something is interfering with their relationship. Temporarily disable any security to rule this in or out as the culprit.

Hopefully, using wireless client mode will help identify where the router relationship has gone sour. Once corrected, you should be able to return to a wireless bridge configuration. Failing all of the above, I suggest chocolate. It won't help the routers at all, but it sure is delicious.

How Can I Control Which Computers Share Files On My Wi-Fi Network?

Q:I have a desktop and a laptop at home which is connected by a wifi router so as to enable Internet sharing. Both run on Windows XP. The desktop is connected via Ethernet while the laptop is connected via Wi-Fi. Also connected to the network are 2 other comps in the same building. How can I enable file sharing only between the 2 computers in my house, so that the files are not accessible by the other 2 computers in the building? – Rohan

A: All of the clients connected to a router share the same subnet. This means that if one machine has shared a folder, the other machine would technically be able to "see" this shared folder.

It is important to make the distinction between "seeing" a shared folder and accessing it. One solution to Rohan's question is to require authentication to access shared folders. This is relatively easy to do in either Windows (XP, Vista/7) and Mac OS X. Depending on the way these folders are accessed, though, this might not be a convenient solution.

Another solution is to divide the LAN into two subnets. You can easily do this using a second router. Just about any wireless router will do. Let's say that we will "cordon off" the Windows XP desktop and laptop described above, so that they can share files.

First, we need to know the IP address of the first router. Using any machine connected to this router, open its administration page and look for the LAN IP address. Common examples would be 192.168.0.1, 192.168.1.1, and 192.168.2.1.

Next, connect a PC to the second router's LAN port using Ethernet. Following the directions in the manual, connect to this router's administration page. If the first three parts of its IP address are the same as that on the first router, we need to change it.

In other words, both routers cannot be 192.168.1.1. More broadly, the second router cannot be 192.168.1.anything. If necessary, assign a different IP address to the second router—consult the examples above. (If the first router is 192.168.1.1, set the second router to either of the alternatives above.) Configure the wireless settings on the second router to whatever network name and security you would like.

Connect an Ethernet cable from a LAN port on the first router to the WAN/Internet port on the second router. Now, PC's connected to the second router (either by wire or wirelessly) can see each other's shared folder. They can also access the Internet thanks to the second router being connected to the first router.

PC's connected directly to the first router can also access the Internet, but won't see shared folders on the machines connected to the second router.

How Can I Share a Hotel Wi-Fi Connection with My Router?

Q:I am on the road and want to share hotel Wi-Fi with another computer in the room plus use Vonage VoIP. Hotel Wi-Fi requires you to pay a fee, create a user name and password through your browser, then enter that user name and password everytime you log on. I have signed on and paid fees for 7 days. I have DD-WRT on a Linksys router with me. My question is how do I get the router to enter the user name and password so that I can have both computers and Vonage plugged into the router. – JM

A: Hotels use a wide variety of authentication systems, often known as "captive portals". With some exceptions, most work in the same fundamental way­—when you connect to the network (either by wire or wirelessly), your machine is assigned an IP address. If your computer's MAC address (a unique serial number assigned to your network adapter) is not in the hotel router's list of allowed machines, you are directed to their login page.

When you log in—either by accepting some terms and/or authorizing payment—your MAC address is then logged in the hotel router's whitelist. Usually, this authorization expires after a certain period of time, after which you will be directed back to the login page.

To use a wireless router in a hotel room using a wired connection, simply connect the hotel room's Ethernet cable to the router's WAN/Internet port. Connect to the router with your computer, and then log in via the portal. The hotel's router will see the MAC address of the router, meaning that additional devices that you use through the router will not be redirected to the login page.

To connect the router to a hotel's Wi-Fi connection is slightly more involved. You would want to configure the router as a wireless client, associating it with the hotel's wireless signal. This does require a router with either DD-WRT or Tomato installed. Once you do this, the same principle applies—log in to the hotel portal using any computer connected to your router, and the router's MAC address will be authorized.

Unfortunately, once your authorization expires, you will need to re-login to the hotel page. There really is no way for the router to perform this login for you, because in most cases the authorization requires user input into a web page.

On a side note, you may experience unpredictable performance using Vonage through a hotel wireless connection. Some hotel firewalls aggressively block ports, which might prevent Vonage from working at all. Many other hotels limit connections to a relatively small slice of bandwidth, which could cause VoIP services to function poorly.

Strangely, I've found that hotel Internet is often more troublesome in higher-priced properties. On the other hand, the beds are usually quite nice.



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