Init commit with many years of arduino sketches and projects. I dont know if the esp8266 includes much, but there are also libraries. I hope they dont have crazy automatic versioning through the Arduino IDE.

Projects/libraries/Installed_libs/Ethernet/examples/UdpReceiveBytewise/UdpReceiveBytewise.pde

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+#include <Ethernet.h>
+#include <UdpBytewise.h>
+
+/* UdpReceiveBytewise.pde: Example how to receive packets over UDP using UdpBytewise library
+ * prints received packet to serial port
+ * bjoern@cs.stanford.edu 12/30/2008
+ */
+
+/* ETHERNET SHIELD CONFIGURATION  
+ * set MAC, IP address of Ethernet shield, its gateway,
+ * and local port to listen on for incoming packets 
+ */
+byte mac[] = { 0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED }; // MAC address to use
+byte ip[] = { 192, 168, 11, 200 }; // Arduino's IP address
+byte gw[] = { 192, 168, 11, 1 };   // Gateway IP address
+unsigned int localPort = 8888; // local port to listen on
+
+#define MAX_SIZE 32 // maximum packet size
+byte packetBuffer[MAX_SIZE]; //buffer to hold incoming packet
+int packetSize; // holds received packet size
+byte remoteIp[4]; // holds recieved packet's originating IP
+unsigned int remotePort; // holds received packet's originating port
+
+int i;
+
+/* SETUP: init Ethernet shield, start UDP listening, open serial port */
+void setup() {
+  Ethernet.begin(mac,ip,gw);
+  UdpBytewise.begin(localPort);
+  Serial.begin(38400); 
+}
+/* LOOP: wait for incoming packets and print each packet to the serial port */
+void loop() {  
+  // if there's data available, read a packet
+  if(packetSize = UdpBytewise.available()) {
+    Serial.print("Received packet of size ");
+    Serial.println(packetSize);
+    
+    UdpBytewise.getSenderIp(remoteIp);
+    Serial.print("From IP ");
+    for(i=0; i<4; i++) {
+      Serial.print(remoteIp[i],DEC);
+      if(i<3) Serial.print(".");
+    }
+    
+    remotePort = UdpBytewise.getSenderPort();
+    Serial.print(" Port ");
+    Serial.println(remotePort); 
+    
+    Serial.println("Contents:");
+    while(UdpBytewise.available()) {
+      Serial.print(UdpBytewise.read(),BYTE);
+    }
+  }
+  //wait a bit
+  delay(10);  
+}