In the first line of sketch #include <ESP8266WiFi.h> we are including ESP8266WiFi library. This library provides ESP8266 specific Wi-Fi routines we are calling to connect to network.

Many examples do not work, i'm on Windows and if try to compile NTPClient.ino all the IDE hang (installed by Board Manager) after many try i found that the include include <WiFiUdp.h> create rpoblem ...