At the end of almost every chapter in this book, we include lists of resources that give further detail on relevant topics. The references are usually either URLs or book references. Unfortunately, URLs tend to change or disappear over time, so if you read this book some time after it has been published and some of the URLs aren't valid anymore, try to use a search engine such as Google to find the updated link. If you still can't find the listed resource, try to look it up in the Internet archive: http://www.archive.org/.
Many chapters refer to the Request For Comments documents (RFCs), which are mirrored by hundreds of Internet sites all around the world and are easy to find. A good starting point is http://www.rfc-editor.org/.