A subnet is a way of taking a single IP network address and locally splitting it up so that this single network IP address can actually be used on several interconnected local networks. Remember, a single IP network number can only be used on a single network.
The important word here is locally: as far as the world outside the machines and physical networks covered by the sub-netted IP network are concerned, nothing whatsoever has changed - it is still just a single IP network. This is important - sub-networking is a local configuration and is invisible to the rest of the world.