The official definition of a WAN is a "communications network that connects geographically separate areas." However, the rub is trying to pin down what constitutes "geographically separate" areas.In general the line between LAN and WAN is crossed once you start using telecommunications systems to connect the various segments of your network. ‘Telecommunication systems’ includes equipment and infrastructure provided and maintained by a third party, typically the phone company. For example, if your network uses communication devices to connect separate segments via public phone, ISDN or T1 lines, then you can probably call it a WAN.  Still, this definition is not perfect. A single employee dialing into a RAS server from his laptop at home doesn't really constitute a WAN.  However, there are few situations where the distinction between a LAN and WAN is so critical that you need to make a precise call one way or the other.

The terms "client" and "server" are used to describe individual computers that are part of a network where computing resources and workload are shared.

• A server is a computer that makes its resources available to the network and responds to the commands of a client. The server’s shared resources can be files (a file server); printers (a print server); processing power (an application server); and just about any computer resource.
• A client is a computer that uses the resources made available by a server. The client must have sufficient processing power on its own to run applications that interact with the resources on the server.

It is possible, and quite common, for an individual computer to function as both a client and a server. For example, if Bill queries a SQL Server database from his workstation for the data he needs to create an Excel spreadsheet, then his workstation is functioning as a client. However, if Sue then connects to Bill’s workstation from her computer and copies the spreadsheet, then Bill’s workstation is functioning as a server.The above discussion focuses on the roles played by individual computers in a network. The concept of extending these roles to create a ‘client/server architecture’ is discussed in another section.

Part of a network's design includes how network resources will be shared and managed. There are two basic network designs: Server-based and peer-to-peer.

• A server-based network is one in which there is a central server.
• A peer-to-peer network also called a workgroup, is a collection of computers that perform both client and server functions. There is no dedicated server or hierarchy among the computers.

In a server-based network, at least one computer is designated as a server for the other computers (clients). Typically the dedicated server is a file server that makes data files available to the clients. However, the server can also provide other resources such as processing time (application server) or printers (print server).The server-based network provides central control of resources, enhanced security and simplifies administration. However, it generally costs more and is less flexible than a peer-to-peer configuration.

Although all Microsoft Windows-based operating systems support peer-to-peer networking, specialized network operating system (NOS) software is required for a server-based network. Several popular NOS are available for a PC network:

UNIX - a popular, robust operating system that comes in several forms from different vendors

Microsoft Windows NT Server– the newest and fastest growing NOS that is very easy to use

Novell NetWare – one of the most popular PC-based NOS

AppleTalk – a NOS developed by Apple that allows communication between Apple computers and other computers that are properly configured

LAN Manager & LAN Server - an older NOS originally developed by Microsoft and IBM that is no longer very common

A server can share almost any type of resource. Some common server roles are:

• File and print sharing servers (e.g., Windows NT Server, Windows 95)
• Database servers (SQL Server)
• Application servers (e.g., accounting packages such as Great Plains Software Dynamics C/S+ or Solomon IV for Windows)
• Web servers (e.g., Internet Information Server)
• Messaging servers (e.g., Exchange Server)
• Specialized servers (e.g., Proxy Server, Index Server, Systems Management Sever, Transaction Server)

Of the examples above, only Windows NT Server is a network operating system. It provides the resources to manage the network, implement security, facilitate communication among processes, etc. The other servers operate ‘on top of’ the network operating system to provide specialized services.

In a peer-to-peer network there is no dedicated server. Instead, each computer acts as both a client and a server. In other words, each computer can share its resources with other computers as well as use the resources of other computers. In order for this arrangement to work, each of the computers on a peer-to-peer network must have enough processing power to support both the client and server functions.

Peer-to-peer configurations are relatively easy to implement and provide a great deal of flexibility. Users can get direct access to resources without going through a dedicated server. Although a peer-to-peer network can implement ‘share-level’ security, it is not as effective as the ‘users-level’ security available with a server-based network. As a general rule, peer-to peer networks are more difficult to administer than a server-based network. Each user must manage his or her own resources because this type of network typically does not have a dedicated system administrator. A peer-to-peer network is sometimes referred to as a ‘workgroup’ or a special case of a client/server network.

Regardless of the type of network, computers must be managed. To facilitate management, computers in a network are organized into groups. With Windows NT, peer-to-peer networks are organized into workgroups. A server-based network is organized as a domain.

• Workgroup. A workgroup is a logical collection of computers identified by a unique name. Members of the workgroup can see and access resources shared by other computers within the group. Each computer in the workgroup manages its own security.

• Domain. A domain is a unique characteristic of a Windows NT server-based network. Like a workgroup, a domain provides a means of organizing computers and resources. Unlike a workgroup, a domain is managed by a central server called a ‘Primary Domain Controller’ or PDC that maintains a single user accounts database. That database can be shared with other servers called ‘Backup Domain Controllers’ (BDCs). The domain controllers provide logon validation to ensure that domain user accounts and security policies are enforced within the domain. Permissions and rights within the domain are typically assigned to groups and then users are assigned to the groups. Thus, a user’s ability to access resources on the network depends primarily on his or her group memberships. There can be more than one domain on a network. Users from one domain can access the resources of another domain if the appropriate trust relationships have been established.

In general, a server-based network is recommended over a peer-to-peer network when:

• A large number of workstations need access a particular resource (e.g., a database).
• Security is a significant concern.
• Adequate administration support is available.
• Projected future growth of the network is planned.
• A heterogeneous environment exists (i.e., Macs and PCs).
• Users are widely dispersed.
• Individual workstations lack the necessary computing power or resources to support a peer-to-peer network.

In general, a peer-to-peer network is favored over a server-based network when:

• Security is not a major concern.
• Number of users is minimal.
• The need for shared resources is minimal.
• There is no dedicated system administrator.
• The projected future growth of the network is limited.
• Users are all located in close proximity.
• Individual workstations possess sufficient computing power and resources.