I've been trying to install an AirPort Express on my existing network.
 My goal is to stream music to Airport Express.

After some work, I was able to get the AirPort Express to successfully
connect to my WLAN (with WPA-PSK enabled).  Then, after I configured
the wireless router to allow both clients (i.e., my laptop and the
Airport Express) to "associate", I was able to ping the IP of the
AirPort Express from my laptop.  But, I still cannot stream music to
the AirPort Express (iTunes starts but doesn't display the option for
AirTunes).  I poked around on Apple's support site and found this
article (http://docs.info.apple.com/article.html?artnum=108071) that
indicates that UDP Port 5353 must not be blocked.

As I don't run any firewall on my laptop, my assumption has been that
the wireless router might be blocking port 5353.

But, in truth, I'm at the limit of my knowledge of IP networking,
ports and routing.  I don't know what the problem is or what to try
next.

The LAN is laid out like this:

ADSL Connenction
      |
   ADSL Modem
      |
  Sitecome Wireless Network Broadband Router WL-114
         |                           |
    WinXP Laptop             Apple Airport Express
   192.168.123.118            192.168.123.199

---

Clarification of Question by pluedog-ga on 14 May 2005 06:05 PDT

I have pinged 224.0.0.1 and the only device that responds is the Apple
Airport express at IP 192.168.123.199.  I get no response from my
laptop or from the router itself (or from an additional laptop I added
to the wireless network).  This is the case regardless of whether WPA
is enabled or not.

Also, I can share music in iTunes between the two laptops.

If you run the AirPort Admin Utility (the one that ends in "Utility",
NOT the one that ends in "Assistant") on your laptop, does it show
your AirPort Express in its list of base stations it can see on the
network?  If it can, then Bonjour (formerly known as Rendezvous)
service discovery is working, so port 5353 isn't the problem (it's
actually Bonjour that uses 5353, but AirTunes makes use of Bonjour).

Another way to test Bonjour would be to put a second wireless PC or
Mac on your wireless network, and run iTunes Music Sharing.  If you
configure iTunes one one of the machines to share its music library,
it advertises it via Bonjour.  So if the other machine can see the
"sharing' machine's iTunes music library on the network, you know your
wireless router isn't breaking Bonjour.

If you determine that Bonjour isn't working on your network, it might
not be a case of port 5353 being blocked.  It could be that your
wireless router has a bug where it doesn't handle "multicast" packets
correctly.  To see if your wireless router handles multicast packets
correctly, ping 224.0.0.1, which is the "all hosts" multicast address.
 Your wireless router AND your AirPort Express should respond to this
ping, causing your ping tool to report multiple (duplicate) responses.
 If you don't see a ping response from both your wireless router and
your AirPort Express, then your wireless router probably has a bug
where it doesn't handle multicasts correctly.

What's strange to me is, I'm pretty sure you can't pass Wi-Fi
certification if you don't handle multicasts correctly, and the
Sitecom WL-114 apparently carries a Wi-Fi logo.  However, I note that
the Wi-Fi Alliance's website (wi-fi.org) doesn't list *any* "Sitecom"
products as being Wi-Fi Certified.  I wonder if this is a clerical
error on the Wi-Fi Alliance's part, or if this is a case of a vendor
trying to pull a fast one.  Both of those possibilities have been
known to happen.

Multicast packet handling is trickier under WPA than it is without
WPA.  So if it comes down to multicasts being broken, you might want
to see if the problem goes away if you turn off security.  That's
obviously not likely to be an acceptable long-term solution, but it
might help for the sake of troubleshooting.

Well that's a strange mix of results. It sounds like multicasts aren't
working reliably on your network, but aren't completely broken either.

Can the AirPort Admin Utility see your AirPort Express?

Do things work better if you turn off wireless security on your network?