What is WAN?

There are two prevailing definitions of a Wide Area Network (WAN). The book definition of a WAN is a network thats spans large georaphical locations, usually to interconnect multiple Local Area Network (LANs)

The Practical Definition of a WAN is a network that traverses a public network or commercial carrier, using one of several WAN tecnologies. Consider the following example.

A connection between two building using ethernet as a medium world generally be considered a LAN. However this is because of the technology used and not the zombie-infested distance between the two buildings.

A connection between the same using a dedicated TI line as a medium would generally be considered a WAN. Remember the difference is the technology used. A variety of WAN technology exist. Each operating at both the physical and data link  layer of the OSI models. Higher layer protocols such as IP are encapsulated when sent across the WAN link.