I would like to comment a bit on the above writeups.
First, blocking a punch. I assume that by blocking, we mean "let the punch hit you on the outside of your arms instead of, say, the head or the solar plexus", as a deflection of the attack has already been mentioned. It might not be as good a solution as to not be there when the punch arrives, but compared to the alternative of having the fist connect to your face, it's a damn good one. It is also fairly easy, as we instinctively try to protect our head. It is not instinctive to step sideways towards an attacker, and the latter requires a bit of training.
Of course, when it comes to winning a fight (that is, walking away without too much injury), just blocking won't help you, as sooner or later, a punch will get through your guard.
On the subject of hit locations to stun the assailant, I will assume that SharQ is talking about the side of the ribcage, not the front. If you've stepped in so that you're in front of your attacker, instead of by his side, go for the groin, nose or solar plexus. The frontal parts of the chest can take hard punches without causing much pain, and if you're like me, and don't have all that much power in your punches, your opponent will probably just laugh and smack you hard in the side of your head with his elbow.
Even if you hit somebody with a lot of force in one of these painful areas, remember that a determined assailant with a large amount of adrenaline in his body (or high on certain drugs, I'm told) might shrug it off like it was nothing, with no seconds-long period of being stunned. I add this as cautionary information.