NodeMCU gets strange IP address - arduino

Question

Today I got my NodeMCU and I instantly started with coding. I wanted to connect to my WiFi and to my MQTT server.
I used the PubSub example for this.
In the serial monitor I get the message that I connected successfully with the WiFi, but I get the IP 172.20.10.6. However we have a 192.168... network.
Then when I try to reach my MQTT server it doesn't find it. When I try to give the NodeMCU a static IP it also says connected successfully and shows up the static IP I gave it, but I still can't connect to my MQTT server.
I can't ping the NodeMCU and don't find it in my Smartphone app "Fing".
#include <ESP8266WiFi.h>
#include <PubSubClient.h>
const char* ssid = "myssid";
const char* password = "mypw";
const char* mqtt_server = "192.168.42.131";
WiFiClient espClient;
PubSubClient client(espClient);
long lastMsg = 0;
char msg[50];
int value = 0;
void setup() {
pinMode(BUILTIN_LED, OUTPUT);
// Initialize the BUILTIN_LED pin as an output
Serial.begin(115200);
setup_wifi();
client.setServer(mqtt_server, 1883);
client.setCallback(callback);
}
void setup_wifi() {
delay(10);
// We start by connecting to a WiFi network
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");
Serial.println("IP address: ");
Serial.println(WiFi.localIP());
}
void callback(char* topic, byte* payload, unsigned int length) {
Serial.print("Message arrived [");
Serial.print(topic);
Serial.print("] ");
for (int i = 0; i < length; i++) {
Serial.print((char)payload[i]);
}
Serial.println();
// Switch on the LED if an 1 was received as first character
if ((char)payload[0] == '1') {
digitalWrite(BUILTIN_LED, LOW);
// Turn the LED on (Note that LOW is the voltage level
// but actually the LED is on; this is because
// it is active low on the ESP-01)
} else {
digitalWrite(BUILTIN_LED, HIGH);
// Turn the LED off by making the voltage HIGH
}
}
void reconnect() {
// Loop until we're reconnected
while (!client.connected()) {
Serial.print("Attempting MQTT connection...");
// Attempt to connect
if (client.connect("ESP8266Client")) {
Serial.println("connected");
// Once connected, publish an announcement...
//client.publish("outTopic", "hello world");
// ... and resubscribe
client.subscribe("mathistest");
} else {
Serial.print("failed, rc=");
Serial.print(client.state());
Serial.println(" try again in 5 seconds");
// Wait 5 seconds before retrying
delay(5000);
}
}
}
void loop() {
if (!client.connected()) {
reconnect();
}
client.loop();
long now = millis();
if (now - lastMsg > 2000) {
lastMsg = now;
++value;
snprintf (msg, 75, "hello world #%ld", value);
}
}

Answer 1

You are most likely having a DHCP failure.
A 172.20.x.x address is a non-routable IP address (see: https://www.lifewire.com/what-is-a-private-ip-address-2625970) and the DHCP code is (likely) using that address when the address assignment fails.
Stepping back, DHCP is most likely failing because you are failing to connect to the Wifi network with the correct SSID and password.