Azure – Does Azure support UDP Broadcast

There is no such thing as an "open" UDP port, at least not in the sense most people are used to think (which is answering something like "OK, I've accepted your connection"). UDP is session-less, so "a port" (read: the UDP protocol in the operating system IP stack) will never respond "success" on its own.

UDP ports only have two states: listening or not. That usually translates to "having a socket open on it by a process" or "not having any socket open". The latter case should be easy to detect since the system should respond with an ICMP Destination Unreachable packet with code=3 (Port unreachable). Unfortunately many firewalls could drop those packets so if you don't get anything back you don't know for sure if the port is in this state or not. And let's not forget that ICMP is session-less too and doesn't do retransmissions: the Port Unreachable packet could very well be lost somewhere on the net.

A UDP port in the "listening" state may not respond at all (the process listening on it just receives the packet and doesn't transmit anything) or it could send something back (if the process does act upon reception and if it acts by responding via UDP to the original sender IP:port). So again, you never know for sure what's the state if you don't get anything back.

You say you can have control of the receiving host: that makes you able to construct your own protocol to check UDP port reachability: just put a process on the receiving host that'll listen on the given UDP port and respond back (or send you an email, or just freak out and unlink() everything on the host file system... anything that'll trigger your attention will do).

Like Brad mentions you can easily connect most Access Points and even consumer Routers to switches and get them to act as simple Access Points. In many cases buying a consumer router and even flashing it with DD-WRT will be much cheaper than buying a specific "Access Point" device and offers much more power and functionality.

So to scale your solution like you asked you simply need to figure out how many clients you can run over a single access point's hardware and software. For the consumer style devices with the standard firmware I wouldn't try to run more than 20. With an alternate firmware like DD-WRT you might be able to pull off 30 max. Your other option is to investigate buying advanced/industrial access points like what are used in large office buildings and schools. These devices can easily support many users, up to thousands of users at once, but they aren't cheap.

Any of the current Linksys or Netgear routers are fairly decent and can provide the connectivity you need. Access points too. I recently purchased two Linksys Wireless-N routers and flashed them with DD-WRT, they support about 40 users total across a single floor of a building and the range is great, especially with DD-WRT sending more power to the antennas.

Typically you just connect a standard ethernet cord from your switch to a switch port on the router or access point and thats all there is to it. Just make sure you observe the proper channel seperation for your locale and for the physical location you are using. Just obtain a free wireless network monitor tool to view the current networks around you and place your devices channels where there isn't much activity.