HA!
f**k me I remember the day we did subnetting during my CCNA exploration course. I think I was one of the few who couldn't grasp it. Took me from 9am when we started the lecture, till midnight when it finally clicked after trying to figure it out all day.
Then I watched Jeremy Nuggets CCNA ICND vids on subnetting and got it figured in the space of 30 mins. FML!!!
I remember OSPF too, no c**t could understand the concept of areas within an AS. Yet I figured it the moment I was taught it....Hardly difficult on ccna level.