Actually, hangover is NOT alcohol withdrawal. To withdraw from a substance, you must have built a dependency which doesn't happen immediately. A first time drinker can end up with a hangover, but they are not in alcohol withdrawal.
Interestingly, a person can develop a dependence on alcohol only drinking one or two drinks a day for a long period of time. I've treated patients who experience mild symptoms of alcohol withdrawal who would drink one or two glasses of wine with dinner, or a couple of beers after work, every single day for years. Their withdrawal is not severe, but it makes them uncomfortable and less than cooperative patients.