Can you link me to the library you want to use?

To include external files you use the Controller options page, which you get to by either double clicking on the controller icon in the project tree, or using the right click menu on that node.

Any file you add a reference to will be added to a local folder in the controller on disk in case you copy the project to another computer, and when you compile a program all of the referenced files are copied into the same folder as the generated code.

I'm not too familiar with using an Ethernet library, but could you use 4 integer inputs who's max range is changed to 999 and put those together in code to create the IP address?

If you can't get this working, send me a link to the library and I'll see if I can put together a node.

If you do get it working, could you send it to me so I can add it to the library? There's a page for that here, but I noticed it needs to be updated with new instructions. Before pasting the node, copy an instance of the node using the right click menu and selecting "copy with references", then check off the referenced files that need to be included.