user63156: I am new to ESP8266 and found an example about turning a LED ON/OFF using web interface. It works well but after some time (few minutes) i cannot open the web page (in my case it is http://192.168.100.23 and i made it static IP). It keeps 'waiting for 192.168.100.23'. I can ping the IP. Can someone please help to find what the problem is? Thank you very much! 
Below is the code:
#include <ESP8266WiFi.h>
#define LED D2
const char* ssid = "myAP";
const char* password = "32133213";
unsigned char status_led=0;
WiFiServer server(80);

void setup() {
  Serial.begin(115200);
  pinMode(LED, OUTPUT);
  /*WiFi.persistent( false );
  WiFi.setAutoConnect(true);
  WiFi.mode(WIFI_STA);*/
  Serial.println();
  Serial.println();
  Serial.print("Connecting to ");
  Serial.println(ssid);
  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) 
  {
    delay(500);
    Serial.print(".");
  }
  Serial.println("");
  Serial.println("WiFi connected");
  server.begin();
  Serial.println("Server started");
  Serial.println(WiFi.localIP());
}

void loop() {
  WiFiClient client = server.available();
  if (!client) {
    return;
  }

  Serial.println("new client");
  while(!client.available())
  {
    delay(1);
  }
  String req = client.readStringUntil('\r');
  Serial.println(req);
  client.flush();
  if (req.indexOf("/ledoff") != -1)
  {
    status_led=0;   
    digitalWrite(LED,LOW);
    Serial.println("LED OFF");
  }
  else if(req.indexOf("/ledon") != -1)
  {
    status_led=1;
    digitalWrite(LED,HIGH);
    Serial.println("LED ON");
  }
  String web = "HTTP/1.1 200 OK\r\nContent-Type: text/html\r\n\r\n";
  web += "<html>\r\n";
  web += "<body>\r\n";
  web += "<h1>LED Status</h1>\r\n";
  web += "<p>\r\n";
  if(status_led==1)
      web += "LED On\r\n";
  else
      web += "LED Off\r\n";
  web += "</p>\r\n";
  web += "</p>\r\n";
  web += "<a href=\"/ledon\">\r\n";
  web += "<button>LED On</button >\r\n";
  web += "</a>\r\n";
  web += "</p>\r\n";

  web += "<a href=\"/ledoff\">\r\n";
  web += "<button>LED Off</button >\r\n";
  web += "</a>\r\n";

  web += "</body>\r\n";
  web += "</html>\r\n";

  client.print(web);
}


user37622: The esp8266 enters modem sleep in STA mode without WiFi traffic. You can turn off the sleep mode with WiFi.setSleepMode. The possible parameters are WIFI_NONE_SLEEP, WIFI_LIGHT_SLEEP or WIFI_MODEM_SLEEP.
After some longer time the servers on esp8266 with arduino core usually get inaccessible even it the esp8266 connects as client to other servers or still sends UDP messages. It is a known issue with some possible workaround documented.

user63156: I added WiFi.setSleepMode(WIFI_NONE_SLEEP); but it does not help. I found another script that is working well so far (after 1 day). I am new so i do not know what problem is but i think it needs to free memory and close connection. Below is the new code for those who get the same problem!
// Load Wi-Fi library
#include <ESP8266WiFi.h>
#define LED D2
// Replace with your network credentials
const char* ssid     = "myAP";
const char* password = "32133213";
// Set web server port number to 80
WiFiServer server(80);
// Variable to store the HTTP request
String header;
// Auxiliar variables to store the current output state
String outputState = "off";
// Assign output variables to GPIO pins

// Current time
unsigned long currentTime = millis();
// Previous time
unsigned long previousTime = 0; 
// Define timeout time in milliseconds (example: 2000ms = 2s)
const long timeoutTime = 2000;
void setup() {
  Serial.begin(115200);
  // Initialize the output variables as outputs
  pinMode(LED, OUTPUT);
  // Set outputs to LOW
  digitalWrite(LED, LOW);

  // Connect to Wi-Fi network with SSID and password
  Serial.print("Connecting to ");
  Serial.println(ssid);
  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }
  // Print local IP address and start web server
  Serial.println("");
  Serial.println("WiFi connected.");
  Serial.println("IP address: ");
  Serial.println(WiFi.localIP());
  server.begin();
}
void loop(){
  WiFiClient client = server.available();   // Listen for incoming clients
  if (client) {                             // If a new client connects,
    Serial.println("New Client.");          // print a message out in the serial port
    String currentLine = "";                // make a String to hold incoming data from the client
    currentTime = millis();
    previousTime = currentTime;
    while (client.connected() && currentTime - previousTime <= timeoutTime) { // loop while the client's connected
      currentTime = millis();         
      if (client.available()) {             // if there's bytes to read from the client,
        char c = client.read();             // read a byte, then
        Serial.write(c);                    // print it out the serial monitor
        header += c;
        if (c == '\n') {                    // if the byte is a newline character
          // if the current line is blank, you got two newline characters in a row.
          // that's the end of the client HTTP request, so send a response:
          if (currentLine.length() == 0) {
            // HTTP headers always start with a response code (e.g. HTTP/1.1 200 OK)
            // and a content-type so the client knows what's coming, then a blank line:
            client.println("HTTP/1.1 200 OK");
            client.println("Content-type:text/html");
            client.println("Connection: close");
            client.println();

            // turns the GPIOs on and off
            if (header.indexOf("GET /on") >= 0) {
              Serial.println("light on");
              outputState = "on";
              digitalWrite(LED, HIGH);
            } else if (header.indexOf("GET /off") >= 0) {
              Serial.println("light off");
              outputState = "off";
              digitalWrite(LED, LOW);
            } 

            // Display the HTML web page
            client.println("<!DOCTYPE html><html>");
            client.println("<head><meta name=\"viewport\" content=\"width=device-width, initial-scale=1\">");
            client.println("<link rel=\"icon\" href=\"data:,\">");
            // CSS to style the on/off buttons 
            // Feel free to change the background-color and font-size attributes to fit your preferences
            client.println("<style>html { font-family: Helvetica; display: inline-block; margin: 0px auto; text-align: center;}");
            client.println(".button { background-color: #195B6A; border: none; color: white; padding: 16px 40px;");
            client.println("text-decoration: none; font-size: 30px; margin: 2px; cursor: pointer;}");
            client.println(".button2 {background-color: #77878A;}</style></head>");

            // Web Page Heading
            client.println("<body><h1>ESP8266 Web Server</h1>");

            // Display current state, and ON/OFF buttons for GPIO 5  
            client.println("<p>State " + outputState + "</p>");
            // If the output5State is off, it displays the ON button       
            if (outputState=="off") {
              client.println("<p><a href=\"/on\"><button class=\"button button2\">TURN ON</button></a></p>");
            } else {
              client.println("<p><a href=\"/off\"><button class=\"button\">TURN OFF</button></a></p>");
            } 

            client.println("</body></html>");

            // The HTTP response ends with another blank line
            client.println();
            // Break out of the while loop
            break;
          } else { // if you got a newline, then clear currentLine
            currentLine = "";
          }
        } else if (c != '\r') {  // if you got anything else but a carriage return character,
          currentLine += c;      // add it to the end of the currentLine
        }
      }
    }
    // Clear the header variable
    header = "";
    // Close the connection
    client.stop();
    Serial.println("Client disconnected.");
    Serial.println("");
  }
}


63156: <p>I am new to ESP8266 and found an example about turning a LED ON/OFF using web interface. It works well but after some time (few minutes) i cannot open the web page (in my case it is <a href="http://192.168.100.23" rel="nofollow noreferrer">http://192.168.100.23</a> and i made it static IP). It keeps 'waiting for 192.168.100.23'. I can ping the IP. Can someone please help to find what the problem is? Thank you very much! </p>

<p>Below is the code:</p>

<pre><code>#include &lt;ESP8266WiFi.h&gt;
#define LED D2
const char* ssid = "myAP";
const char* password = "32133213";
unsigned char status_led=0;
WiFiServer server(80);

void setup() {
  Serial.begin(115200);
  pinMode(LED, OUTPUT);
  /*WiFi.persistent( false );
  WiFi.setAutoConnect(true);
  WiFi.mode(WIFI_STA);*/
  Serial.println();
  Serial.println();
  Serial.print("Connecting to ");
  Serial.println(ssid);
  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) 
  {
    delay(500);
    Serial.print(".");
  }
  Serial.println("");
  Serial.println("WiFi connected");
  server.begin();
  Serial.println("Server started");
  Serial.println(WiFi.localIP());
}

void loop() {
  WiFiClient client = server.available();
  if (!client) {
    return;
  }

  Serial.println("new client");
  while(!client.available())
  {
    delay(1);
  }
  String req = client.readStringUntil('\r');
  Serial.println(req);
  client.flush();
  if (req.indexOf("/ledoff") != -1)
  {
    status_led=0;   
    digitalWrite(LED,LOW);
    Serial.println("LED OFF");
  }
  else if(req.indexOf("/ledon") != -1)
  {
    status_led=1;
    digitalWrite(LED,HIGH);
    Serial.println("LED ON");
  }
  String web = "HTTP/1.1 200 OK\r\nContent-Type: text/html\r\n\r\n";
  web += "&lt;html&gt;\r\n";
  web += "&lt;body&gt;\r\n";
  web += "&lt;h1&gt;LED Status&lt;/h1&gt;\r\n";
  web += "&lt;p&gt;\r\n";
  if(status_led==1)
      web += "LED On\r\n";
  else
      web += "LED Off\r\n";
  web += "&lt;/p&gt;\r\n";
  web += "&lt;/p&gt;\r\n";
  web += "&lt;a href=\"/ledon\"&gt;\r\n";
  web += "&lt;button&gt;LED On&lt;/button &gt;\r\n";
  web += "&lt;/a&gt;\r\n";
  web += "&lt;/p&gt;\r\n";

  web += "&lt;a href=\"/ledoff\"&gt;\r\n";
  web += "&lt;button&gt;LED Off&lt;/button &gt;\r\n";
  web += "&lt;/a&gt;\r\n";

  web += "&lt;/body&gt;\r\n";
  web += "&lt;/html&gt;\r\n";

  client.print(web);
}
</code></pre>
 37622: <p>The esp8266 enters modem sleep in STA mode without WiFi traffic. You can turn off the sleep mode with WiFi.<a href="https://arduino-esp8266.readthedocs.io/en/latest/esp8266wifi/generic-class.html#wifi-power-management-dtim" rel="nofollow noreferrer">setSleepMode</a>. The possible parameters are WIFI_NONE_SLEEP, WIFI_LIGHT_SLEEP or WIFI_MODEM_SLEEP.</p>

<p>After some longer time the servers on esp8266 with arduino core usually get inaccessible even it the esp8266 connects as client to other servers or still sends UDP messages. It is a <a href="https://github.com/esp8266/Arduino/issues/2330" rel="nofollow noreferrer">known issue</a> with some possible workaround documented.</p>
 63156: <p>I added WiFi.setSleepMode(WIFI_NONE_SLEEP); but it does not help. I found another script that is working well so far (after 1 day). I am new so i do not know what problem is but i think it needs to free memory and close connection. Below is the new code for those who get the same problem!</p>

<pre><code>// Load Wi-Fi library
#include &lt;ESP8266WiFi.h&gt;
#define LED D2
// Replace with your network credentials
const char* ssid     = "myAP";
const char* password = "32133213";
// Set web server port number to 80
WiFiServer server(80);
// Variable to store the HTTP request
String header;
// Auxiliar variables to store the current output state
String outputState = "off";
// Assign output variables to GPIO pins

// Current time
unsigned long currentTime = millis();
// Previous time
unsigned long previousTime = 0; 
// Define timeout time in milliseconds (example: 2000ms = 2s)
const long timeoutTime = 2000;
void setup() {
  Serial.begin(115200);
  // Initialize the output variables as outputs
  pinMode(LED, OUTPUT);
  // Set outputs to LOW
  digitalWrite(LED, LOW);

  // Connect to Wi-Fi network with SSID and password
  Serial.print("Connecting to ");
  Serial.println(ssid);
  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }
  // Print local IP address and start web server
  Serial.println("");
  Serial.println("WiFi connected.");
  Serial.println("IP address: ");
  Serial.println(WiFi.localIP());
  server.begin();
}
void loop(){
  WiFiClient client = server.available();   // Listen for incoming clients
  if (client) {                             // If a new client connects,
    Serial.println("New Client.");          // print a message out in the serial port
    String currentLine = "";                // make a String to hold incoming data from the client
    currentTime = millis();
    previousTime = currentTime;
    while (client.connected() &amp;&amp; currentTime - previousTime &lt;= timeoutTime) { // loop while the client's connected
      currentTime = millis();         
      if (client.available()) {             // if there's bytes to read from the client,
        char c = client.read();             // read a byte, then
        Serial.write(c);                    // print it out the serial monitor
        header += c;
        if (c == '\n') {                    // if the byte is a newline character
          // if the current line is blank, you got two newline characters in a row.
          // that's the end of the client HTTP request, so send a response:
          if (currentLine.length() == 0) {
            // HTTP headers always start with a response code (e.g. HTTP/1.1 200 OK)
            // and a content-type so the client knows what's coming, then a blank line:
            client.println("HTTP/1.1 200 OK");
            client.println("Content-type:text/html");
            client.println("Connection: close");
            client.println();

            // turns the GPIOs on and off
            if (header.indexOf("GET /on") &gt;= 0) {
              Serial.println("light on");
              outputState = "on";
              digitalWrite(LED, HIGH);
            } else if (header.indexOf("GET /off") &gt;= 0) {
              Serial.println("light off");
              outputState = "off";
              digitalWrite(LED, LOW);
            } 

            // Display the HTML web page
            client.println("&lt;!DOCTYPE html&gt;&lt;html&gt;");
            client.println("&lt;head&gt;&lt;meta name=\"viewport\" content=\"width=device-width, initial-scale=1\"&gt;");
            client.println("&lt;link rel=\"icon\" href=\"data:,\"&gt;");
            // CSS to style the on/off buttons 
            // Feel free to change the background-color and font-size attributes to fit your preferences
            client.println("&lt;style&gt;html { font-family: Helvetica; display: inline-block; margin: 0px auto; text-align: center;}");
            client.println(".button { background-color: #195B6A; border: none; color: white; padding: 16px 40px;");
            client.println("text-decoration: none; font-size: 30px; margin: 2px; cursor: pointer;}");
            client.println(".button2 {background-color: #77878A;}&lt;/style&gt;&lt;/head&gt;");

            // Web Page Heading
            client.println("&lt;body&gt;&lt;h1&gt;ESP8266 Web Server&lt;/h1&gt;");

            // Display current state, and ON/OFF buttons for GPIO 5  
            client.println("&lt;p&gt;State " + outputState + "&lt;/p&gt;");
            // If the output5State is off, it displays the ON button       
            if (outputState=="off") {
              client.println("&lt;p&gt;&lt;a href=\"/on\"&gt;&lt;button class=\"button button2\"&gt;TURN ON&lt;/button&gt;&lt;/a&gt;&lt;/p&gt;");
            } else {
              client.println("&lt;p&gt;&lt;a href=\"/off\"&gt;&lt;button class=\"button\"&gt;TURN OFF&lt;/button&gt;&lt;/a&gt;&lt;/p&gt;");
            } 

            client.println("&lt;/body&gt;&lt;/html&gt;");

            // The HTTP response ends with another blank line
            client.println();
            // Break out of the while loop
            break;
          } else { // if you got a newline, then clear currentLine
            currentLine = "";
          }
        } else if (c != '\r') {  // if you got anything else but a carriage return character,
          currentLine += c;      // add it to the end of the currentLine
        }
      }
    }
    // Clear the header variable
    header = "";
    // Close the connection
    client.stop();
    Serial.println("Client disconnected.");
    Serial.println("");
  }
}
</code></pre>
