Having read this chapter, you should now have a very good understanding of the gopacket package. You should be able to write a simple packet-capturing application using the examples from this chapter. Once again, it is not about memorizing all of the functions or the details about the layers. The important thing is to understand the big picture at a high level and be able to recall what tools are available to you when scoping and implementing an application.
Try writing your own program based on these examples to capture interesting network traffic from your machine. Try capturing and inspecting a specific port or application to see how it works over the wire. See the difference between applications that use encryption and ones that pass data over the wire in plaintext. You may just want to capture all the traffic going on in the background and see which applications are...