When I was at university I wanted to take a class that had 'introduction to computer networks' as a prerequisite, but I didn't want to wait 2 semesters for it to come around again after taking networks first. I spoke to the professor and he said he'd let me enroll in the course if I promised to carefully read Beej's guide first, since it contains all the essentials I'd really need for this follow-on. So, this saved me 6 months, what a golden nugget!