Area 51's origin?

So, how was it designated Area 51? Is this a numeric coding system applied by the government or attributed it through outsiders? If this is a govt code system are there 50 other areas?

Actually there are: I looked it up and Areas 1-30 are Numbered areas around Nellis the base connected to Area 51. Basically blocks of government land. It ws designated Area 51 in 1958 by ther Atomic Energy Commission. But there are some that say the origins of the name are classified.