user56368: I'm going to create an IoT system. I use a Raspberry Pi as a MQTT broker and as a webserver. Every day, the Pi gets a new IP address, so I need to find its new IP address via ESP8266 to use this information in my code. I know that I can use a fixed IP, but I don't want to, so it is better if I can find a solution.

user4143: You should use mDNS.  Install (if it isn't already) avahi-daemon on your Raspberry. 
Then check out the ESP8266mDNS library and the examples it has. Especially the mDNS-SD_Extended example, which performs a query for a service on the network.

56368: <p>I'm going to create an IoT system. I use a Raspberry Pi as a MQTT broker and as a webserver. Every day, the Pi gets a new IP address, so I need to find its new IP address via ESP8266 to use this information in my code. I know that I can use a fixed IP, but I don't want to, so it is better if I can find a solution.</p>
 4143: <p>You should use mDNS.  Install (if it isn't already) <code>avahi-daemon</code> on your Raspberry. </p>

<p>Then check out the ESP8266mDNS library and the examples it has. Especially the mDNS-SD_Extended example, which performs a query for a service on the network.</p>
