If you are a network administrator, and you are setting up routing protocols, if you are using your OpenBSD box as a router, if you need to go in dept

http://www.3com.com/other/pdfs/infra/corpinfo/en_US/501302.pdf
http://openbsd.org/faq/faq6.html#Intro
If you are working with applications such as web servers, ftp servers, and mail servers, you may benefit greatly by reading the RFCs.
6.1 - Before we go any further

For the bulk of this document, it helps if you have read and at least partially understood the Kernel Configuration and Setup section of the FAQ, and the ifconfig(8) and netstat(1) man pages.

If you are a network administrator, and you are setting up routing protocols, if you are using your OpenBSD box as a router, if you need to go in depth into IP networking, you really need to read Understanding IP Addressing. This is an excellent document. "Understanding IP Addressing" contains fundamental knowledge to build upon when working with IP networks, especially when you deal with or are responsible for more than one network.

If you are working with applications such as web servers, ftp servers, and mail servers, you may benefit greatly by reading the RFCs. Most likely, you can't read all of them. Pick some topics that you are interested in, or that you use in your network environment. Look them up, find out how they are intended to work. The RFCs define many (thousands of) standards for protocols on the Internet and how they are supposed to work.