The rest of the stack. Spend some time with layers 3&4 and you won't regret it. It was the most boring class I has in school and I use it every day. Read this book: http://goo.gl/ZGifKa (Internetworking with TCP/IP Vol.1: Principles, Protocols, and Architecture (4th Edition)).

Knowing how TCP/IP works will give you an edge over other devs for understanding data movement and endear you to the IT/network/security folks (who do you favors).