There are two ways you can find the IP address for your 851N.
The first is by pressing the 'i' button on the front of the 851N. Your IP address will then be listed alongside other information such as Unit ID.
The second way is within the Cambridge Audio Connect app. The IP address can be found on an iOS device by clicking on the info button, next to your device in the Players Section. On an Android device, you will need to click the 3 dots which appear to the left of your device in the Players Section. This will then open a menu allowing you to select 'Device Info'.