Please use code tags (</> button on the toolbar) when you post code or warning/error messages. The reason is that the forum software can interpret parts of your code as markup, leading to confusion, wasted time, and a reduced chance for you to get help with your problem. This will also make it easier to read your code and to copy it to the IDE or editor. If your browser doesn't show the posting toolbar then you can just manually add the code tags:
[code]``[color=blue]// your code is here[/color]``[/code]
Using code tags and other important information is explained in the How to use this forum post. Please read it.
To fix the error, add the following near the top of your sketch:
#include <IPAddress.h>
When using other network libraries, you might notice it's not necessary to do this. That is because those libraries contain an #include directive for this file.