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Modern networks exist at drastically varying sizes, and it is useful to categorise them into three main classes according to their scale. The three most commonly used terms to describe the scope or scale of a particular network are the local area network (LAN), metropolitan area network (MAN), and wide area network (WAN), in order of their increasing geographical size. Although there are no real hard and fast standards that dictate the size a specific network must be to qualify as a specific scale denoted by one of these terms, it is usually not difficult to come to an agreement on terms to use.

In many cases, there is an hierarchical relationship between these three grades of network scale. One LAN may be combined with many others in a metropolitan area and can be interconnected to form a MAN; and one or more MANs across multiple metropolitan areas may be interconnected to form a WAN. On the other hand, networks at any of these three scales could be created as single networks. This choice is driven by a combination of business and technical factors depending on the individual parties and technologies involved in a specific area and is not prescribed by the terms themselves.

To show each of these network types visually, consider the diagram in Figure 3.3, which shows two cities:

Figure 3.3 LAN, MAN, and WAN networks.

The diagram shows multiple LANs within a city, a single MAN covering all of that city, and a WAN that is connecting the two cities to each other despite them being 100 miles apart. Of course, a city may have hundreds or thousands of LANs, and there may be multiple MANs and also multiple WANS in that area or between areas; but this example serves to show the difference in scale between typical networks in each of these categories and how one may appear to nest within another from above.

There is also not necessarily any direct hierarchical relationship between these network categories at all. For example, depending on the network topology in a particular area, a LAN may just connect directly to a WAN. In another, it may need to connect to a MAN, which itself connects to a WAN. The specifics in this regard are location and implementation choices made by those network operators.

As well as a different physical scale, the purposes of each of these types of networks are different from one another. A LAN is typically used to connect endpoints within a single building or campus together or to other network resources, whereas a MAN is often used to connect multiple LANs to each other across an area such as a city. A WAN, then, is typically used to connect one network or endpoint to a network resource that is a significant distance away, hence its “wide area” naming.