ESP8266 Community Forum

Thanks Martin.
the next version will fix this problem.

Am I correct in thinking that UDP can only work within a LAN, i.e. is there any way to get this to work with ESP8266's that are on separate networks (different cities, even)?

No, it can works also on separate networks. You need to configure your local router in order to open the UDP port.

Ok so we have UDP broadcast packets (255's) that all can see if they are tuned to UDP and a specific port. The message has some features of publish subscribe of MQTT. Here the UDP packet has the subject as a guaranteed part of the packet and the receiver decides to accept and interact or reject based on the subject. Further the broadcaster can based on the reply of the receiver open a private channel based on the receiver of the broadcast's unique IP. This is very flexible and very useful. The only feature not offered is the store and forward that MQTT has.
Any esp8266 with UDP can be a broadcaster and any esp8266 can be a receiver somewhat like the hardwired CAN bus.
Nice