failed to get an IP address using DHCP Arduino - arduino

Question

I have ethernet shield w5100. Second day I try connecting to internet. But have no result.
My example:
#include <SPI.h>
#include <Ethernet.h>
byte mac[] = { 0x00, 0xAA, 0xBB, 0xCC, 0xDE, 0x01 };
IPAddress ip(192,168,1,20);
EthernetClient client;
const unsigned long requestInterval = 60000; // delay between requests
char serverName[] = "api.twitter.com"; // twitter URL
boolean requested;
unsigned long lastAttemptTime = 0;
String currentLine = ""; // string to hold the text from server
String tweet = ""; // string to hold the tweet
boolean readingTweet = false;
void setup() {
currentLine.reserve(256);
tweet.reserve(150);
// Open serial communications and wait for port to open:
Serial.begin(9600);
// attempt a DHCP connection:
Serial.println("Attempting to get an IP address using DHCP:");
if (!Ethernet.begin(mac)) {
// if DHCP fails, start with a hard-coded address:
Serial.println("failed to get an IP address using DHCP, trying manually");
Ethernet.begin(mac, ip);
}
Serial.print("My address:");
Serial.println(Ethernet.localIP());
// connect to Twitter:
connectToServer();
}
void loop()
{
if (client.connected()) {
if (client.available()) {
// read incoming bytes:
char inChar = client.read();
// add incoming byte to end of line:
currentLine += inChar;
// if you get a newline, clear the line:
if (inChar == '\n') {
currentLine = "";
}
// if the current line ends with <text>, it will
// be followed by the tweet:
if ( currentLine.endsWith("<text>")) {
// tweet is beginning. Clear the tweet string:
readingTweet = true;
tweet = "";
}
// if you're currently reading the bytes of a tweet,
// add them to the tweet String:
if (readingTweet) {
if (inChar != '<') {
tweet += inChar;
}
else {
// if you got a "<" character,
// you've reached the end of the tweet:
readingTweet = false;
Serial.println(tweet);
// close the connection to the server:
client.stop();
}
}
}
}
else if (millis() - lastAttemptTime > requestInterval) {
// if you're not connected, and two minutes have passed since
// your last connection, then attempt to connect again:
connectToServer();
}
}
void connectToServer() {
// attempt to connect, and wait a millisecond:
Serial.println("connecting to server...");
if (client.connect(serverName, 80)) {
Serial.println("making HTTP request...");
// make HTTP GET request to twitter:
client.println("GET /1/statuses/user_timeline.xml?screen_name=arduino& count=1 HTTP/1.1");
client.println("HOST: api.twitter.com");
client.println("Connection: close");
client.println();
}
// note the time of this connect attempt:
lastAttemptTime = millis();
}
OUTPUT = failed to get an IP address using DHCP, trying manually
(connect to switch)
How can I resolve this problem?