Address space is the memory allocated for all possible addresses for a computational entity, such as a device, a file, a server, or a networked computer. Address space may refer to a range of either physical or virtual addresses accessible to a processor or reserved for a process. As unique identifiers of single entities, each address specifies an entity's location (unit of memory that can be addressed separately).
Memory addresses in flat address space are expressed starting at zero and continuing as as incrementally increasing integers one, two ,three etc until the end of memory space. Each address represents a unit of memory.
Flat address space is used in 32-bit computing to address space up to 4 gigabytes (GB). Segmented memory addressing, the alternative to the flat scheme, is used in earlier 16 bit systems such as MS-DOS Windows 3.1., to address 64KB segments of memory with an offset to specify within the segment. Strictly speaking, flat address space is actually segmented but the entire space is one segment addressing the entirety of the 32-bit segment or 4GB memory space.