Arduino Webserver for W5100 Shield

For Nostalgia’s sake: An Arduino Uno with a W5100 ethernetshield, serving a webpage with results of a DS18N20

#include <Ethernet.h> // Library for W5100-based Ethernet shield
#include <OneWire.h>  // Library for OneWire communication
#include <DallasTemperature.h> // Library for DS18B20 temperature sensor

// Pin where the DS18B20 is connected
#define ONE_WIRE_BUS 2
OneWire oneWire(ONE_WIRE_BUS);
DallasTemperature sensors(&oneWire);

EthernetServer server(80); // Ethernet server on port 80

void setup() {
  // Start the serial communication for debugging
  Serial.begin(9600);

  // Set up the Ethernet shield with a fixed MAC address and IP address
  byte mac[] = { 0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED }; // MAC address
  IPAddress ip(192, 168, 1, 177); // Local IP address (adjust as needed for your network)

  // Start the Ethernet and the server
  Ethernet.begin(mac, ip);
  server.begin();
  Serial.print("Server is at ");
  Serial.println(Ethernet.localIP());

  // Start the temperature sensor
  sensors.begin();
}

void loop() {
  // Check for incoming client connections
  EthernetClient client = server.available();
  if (client) {
    Serial.println("New client");

    // Wait for data from the client
    boolean currentLineIsBlank = true;
    while (client.connected()) {
      if (client.available()) {
        char c = client.read();

        // If the request is complete (double newline), send the response
        if (c == '\n' && currentLineIsBlank) {
          // Get the temperature reading
          sensors.requestTemperatures(); // Request temperature
          float temperature = sensors.getTempCByIndex(0); // Get temperature in Celsius

          // Send the HTTP response headers
          client.println("HTTP/1.1 200 OK");
          client.println("Content-Type: text/html");
          client.println("Connection: close"); // The connection will close after this response
          client.println();

          // Send the HTML content
          client.println("<!DOCTYPE HTML>");
          client.println("<html>");
          client.println("<h1>Temperature Monitor</h1>");
          client.print("<p>Temperature: ");
          client.print(temperature);
          client.println(" °C</p>");
          client.println("</html>");
          break;
        }

        if (c == '\n') {
          currentLineIsBlank = true;
        } else if (c != '\r') {
          currentLineIsBlank = false;
        }
      }
    }

    // Close the connection with the client
    client.stop();
    Serial.println("Client disconnected");
  }
}

This sketch initializes the Ethernet shield with a MAC address and a static IP address. Adjust the IP address and MAC address to fit your network configuration.
It creates an Ethernet server listening on port 80 for HTTP requests.
The sketch initializes the DS18B20 temperature sensor on a OneWire bus (here on digital pin 2).
When a client connects, the code reads from the temperature sensor and serves a simple HTML page with the temperature in degrees Celsius.
The connection to the client is closed after sending the HTTP response.

With this sketch, the Ethernet shield serves as a basic web server that provides temperature data from the DS18B20 sensor. Before uploading, ensure the Ethernet, OneWire, and DallasTemperature libraries are installed in your Arduino IDE.