Ask the Wi-Fi Guru, Episode XXV

By Aaron Weiss

March 04, 2010

Our monthly Q&A series offers advice to those seeking help with home or small business WLANs. This month our guru helps troubleshoot a WDS setup, pick the best dd-wrt firmware for a Linksys router and turns to his loyal readers for help with a public hotspot configuration problem.

Our monthly Q&A series offers advice to those seeking help with home or small business WLANs. This month our guru helps troubleshoot a WDS setup, pick the best dd-wrt firmware for a Linksys router and turns to his loyal readers for help with a public hotspot configuration problem.


When you're out and about, particularly if traveling in a foreign city, sometimes the two most exciting words you can see are "free Wi-Fi" (second only to "public restroom"). Typically the domain of coffee shops and bookstores, and sometimes city centers and libraries, finding a free wireless connection can be as refreshing as locating water in the desert. Not that I am suggesting that checking e-mail is as important as water. It's even better!

A new proposed bill in the UK could put an end to free hotspots in the guise of protecting copyright. The logic seems to be that providers of Internet access share responsibility in what activities users engage in. Because users may get involved in copyright-violating actions, best not to provide access at all. Not that this "logic" seems very logical at all. However, it does raise the interesting question of whether owners of open, unsecured wireless connections are responsible for how they are used.

Why Won't My WDS Router Let Me Connect to the Internet?

Q: My WDS router will not allow me to connect to the Internet. I am actually able to connect to the administration page of the WDS router through my primary router to check all the settings. I notice that when I run IP config when on the WDS, there is no "connecton-specific DNS suffix" and no default gateway established. Any suggestions? — Rob

A: Often, the trouble with a WDS configuration is in the link between the chained routers. In this case, it sounds like your two routers are successfully linked. Because you can connect to the administration page of the secondary router when associated with the primary router, the link is in place.

However, when you connect a PC to the secondary router, it is not receiving an IP address—hence the lack of an assigned gateway. When your PC requests an address via DHCP, it also receives a netmask and gateway address provided by the DHCP server running on the router.

Two possible problems come to mind. One, double-check that your PC is actually set to automatically receive an IP address. This is the default setting for network connections in Windows and Mac OS X, but if you previously manually configured your network connection (perhaps while configuring the routers), be sure to reset it to automatic.

Second, be sure that the DHCP server is disabled on the secondary router. There must be only one DHCP server running on your network, and that is on the primary router. Your secondary router's DHCP server is enabled by default—unless you disable it, any PC connected to the secondary router will receive an incorrect assignment.

How Can I Tell Which Linksys Router Is Best for dd-wrt?

Q: In the article How to Choose the Best WRT54G Router for You, you said that the Linksys WRT54GS Version 3 is the best for power users with the DD-WRT firmware. My question is what is the serial numbers for version 2 and version 3 of the Linksys WRT54GS router? I bought a WRT54GS router in 2005 and its serial number begins with CGN4. Also what is the best DD-WRT firmware version to use for security and functionality on the WRT54GS router? — Brian

A: Because there have been many versions of WRT54G router released over the years, and several versions of DD-WRT, you can use the serial number of your router to determine its version. You can use its version to determine which DD-WRT download to use.

The three most popular lines of WRT54G router are the WRT54G, WRT54GS, and WRT54GL. The "GL" models are all Linux-based and are compatible with the full version of DD-WRT. Consult this chart to translate the router's serial number to version number:

WRT54G WRT54GS
Router version Serial number Router version Serial number
1.0 CDF0,CDF1 1.0 CGN0,CGN1
1.1 CDF2,CDF3 1.1 CGN2
2.0 CDF5 2.0 CGN3
2.1 CDF6 2.1 CGN4
2.2 CDF7 3.0 CGN5
3.0 CDF8 4.0 CGN6
3.1 CDF9 5.0 CGN7
4.0 CDFA 5.1 CGN8
5.0 CDFB 6.0 CGN9
5.1 CDFC 7.0 CGNA,CGNB,CGNC
5.2 CDFB 7.2 CGNE
6.0 CDFD
7.0 CDFE
7.2 CDFK
8.0 CDFF,CDFG
8.1 MDF0
8.2 CDFJ

Looking at the chart, your CGN4 WRT54GS is a version 2.1 router. Visit the DD-WRT Router Database and type in "wrt54gs" to see a list of downloads compatible with each version of your router. (In this case, click the entry for router version 2.1)

With rare exceptions, you should choose the latest version of DD-WRT available (currently V24 SP1). This will usually be the most secure and feature-rich version available. Occasionally a new bug will affect a feature, making an older version a better choice for a specific usage, in which case we will do our best to mention this in our DD-WRT coverage.

How Do I Put Wi-Fi Hotspot Users on a Waiting List?

Q:I have been looking for a feature in commercial grade hotspot WiFi access points and have been getting funny looks from vendors and sales people. I have a Wi-Fi-enabled public hotspot and have deployed various access points from different vendors over the past several years and find one major design flaw.

I configure the access access points to no more than perhaps 25 users. BUT anytime I set a maximum number of users what happens to the 26th person who attempts to associate their laptop with my AP? None of the APs out there seem to have a nice, user friendly way to deal with that. Not one sales person across the landscape could answer that question. Seems to me that if I were writing the firmware, I would reserve 6 or so IP addresses, and when the 26th person asked for an IP address, I would have the AP assign one of these standby IP's, and flash the user a friendly page "Sorry, we are busy right now, try again later" of "Sorry we are busy right now, stand by, you are 1st in the queue." The current status is that when the 26th person tries to associate, they fail and then come complain to my counter staff that the Wi-Fi is not working. If you have heard of this before and have an existing solution that I have never heard of ... please let me know. - Jim

A: Great question! When I first read Jim's submission, my gut feeling was that there must be a simple solution to what seems like a very common hotspot situation. But like Jim, I have so far drawn a blank trying to find an answer.

There are many routers that let you specify a maximum number of clients. But this puts the router into a catch-22: if there is a maximum of 25 clients, the 26th person is not assigned an IP address. And so the router has no way to deliver a web page to that person, saying in effect that the queue is full.

And so I'd like to invite the Wi-Fi Planet readership to weigh in: how would you solve Jim's conundrum? Is there a commercial product on the market that already sanely handles this (seemingly common) situation? A DIY solution? Let's hear it!



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