How can an external device send UDP packets to a CueServer?
Nearly any computer (PC, Mac or other) can create and transmit UDP packets through it's Ethernet port (or via it's wireless connection), which can be sent to one or more CueServer devices connected to the network.
There are many ways of creating and sending UDP packets from all kinds of operating systems. In general, you can use a programming language like C, C++ or something similar to make calls to the WinSock (Windows) or BSD Sockets (Mac OS, Linux and others) system functions (bind(), connect() and send(), etc.).
Wikipedia has a good article on User Datagram Protocol (UDP), which contains an overview, and contains some programming examples. Also of note, at the bottom of the Wikipedia article, there is mention of using the Ruby scripting language to send UDP messages, which is perhaps easier to write code for, and should run on nearly any Windows, Mac OS X, Linux and other mainstream computer.
To remotely control CueServer via Ethernet messages, create a UDP packet that contains a plain-text CueScript command, like "Cue 1 Go". Then, send the packet to the IP Address of the CueServer at port number 52737 (0xCE01). When the CueServer receives the packet, it will attempt to execute the CueScript.
You can also send UDP packets to multiple CueServers at the same time by using CueServer's multicast address, which is 126.96.36.199. Any UDP packets sent to this address will be received by ALL CueServers on the local network.
You can use the CueScript "Device" command to specify that certain CueScript statements are only executed by specific CueServer devices when broadcasting UDP CueScript messages to the entire network. See the CueServer User's Manual for additional information.