A client is a software component that enables a computer to access the resources provided by a server. Clients can take many forms, and they can either be included as part of an operating system or distributed as a separate product. In its simplest form, a client can be a stand-alone program that sends requests to and receives replies from a server. Your Web browser, for example, is a client that communicates with Web servers on your local network or the Internet. In the same way, FTP, e-mail, and newsreader programs are all clients. These clients function at the application layer of the OSI reference model and are highly specialized; they only communicate with one type of server. Application layer clients contain no lower layer protocols of their own, relying instead on protocols such as TCP, IP, and Ethernet, which are already installed on the computer, to provide network communication services.
The other main type of client on a workstation is the one that enables you to access shared resources on the local network, such as files and printers. This type of client is more tightly integrated with the operating system; you don't have to launch a special program and you can access files and printers through your regularapplications, just as if they were part of your local computer environment. This type of client is specific to the platform used by the server. There are clients for Windows networks, clients for NetWare, and clients for UNIX systems. In some cases, the client is supplied as part of the operating system, whereas in others you must install a separate client software package. The following sectionsexamine the different LAN client platforms.
Almost all versions of Windows (including Microsoft Windows for Workgroups, Microsoft Windows 95, Microsoft Windows 98, Microsoft Windows Me, and Windows NT and Windows 2000, in both Server and Workstation versions) include both client and server capabilities with the operating system. This means that you can share the files and printers on any of these Windows systems and also use the client capabilities to access shared files and printers on other computers. Note that Windows 3.1 and earlier versions ship with no network client at all.
In the case of Windows 95, Windows 98, Windows Me, Windows NT, and Windows 2000, the operating system includes everything you need to connect to a Windows network, including a complete client networking stack. The stack, shown inFigure 4.4, consists of the following major components:
Figure 4.4 The Windows networking stack consists of several components that work together to provide client access to network resources
Together with the network interface adapter, these software components provide the functions of all seven layers of the OSI model. A system can have more than one of each component installed, providing alternative paths through the networking stack for different applications. Most of the Windows operating systems include two redirectors; for example, there might be one for Windows networking and one for connecting to NetWare servers. The operating systems include multiple protocol drivers for the same purpose. NetWare connectivity traditionally requires the IPX protocol (although the latest versions of NetWare do support TCP/IP), and a Windows network can use TCP/IP or NetBEUI. Windows and NetWare systems usually share the same network medium. It's also possible to install two network interface adapters, each with its own driver, and connect the computer to two networks, one for Windows and one for NetWare, but this is not often done.
Although the drivers can take different forms, all of the Windows operating systems contain the same set of basic networking components, with the exception of Windows for Workgroups. Windows for Workgroups was developed in the early days of Microsoft networking and is rarely used today. That operating system includes a redirector for Windows networking and the NetBEUI and IPX protocols, but no NetWare client is included, nor is the TCP/IP protocol. However, you can add NetWare support by installing a client supplied by Novell, and you can add TCP/IP support by downloading and installing the TCP/IP-32 update, available from Microsoft at ftp://ftp.microsoft.com/peropsys/windows/public/tcpip/WFWT32.EXE/.
The protocols at the various layers specify the path up or down through the OSI model. For example, when a packet arrives at a workstation from the network, the Ethernet frame contains a code that specifies the network layer protocol that it should use. The network layer protocol header then specifies a transport layer protocol, and the transport layer header contains a port number that identifies the application that should receive the data. For packets generated by the workstation, the process works in reverse. The redirector specifies a transport layer protocol, the transport layer specifies the network layer protocol, and the network layer specifies the data-link layer protocol.
Although technologies like Plug and Play now automate the installation of the Windows networking components in most cases, you may sometimes find yourself installing a client or a protocol manually. The process of installing a protocol module on a Windows 2000 Professional system is described next. The procedure is virtually identical in Windows 95, Windows 98, and Windows Me. Windows NT uses a slightly different user interface, but the networking architecture usesessentially the same components.
Figure 4.5 The Local Area Connection Properties dialog box lists all of the networking components installed on the system
The only networking component that you do not install from the Network And Dial-Up Connections dialog box is the network interface adapter driver,because this module is associated with the network interface adapter in the system. To install a network interface adapter driver manually, use the Add/Remove Hardware tool found in Control Panel.
When Microsoft first introduced its own network operating systems (Windows for Workgroups and Windows NT) in 1993, Novell NetWare ruled the local area networking industry. To successfully compete with Novell, Microsoft knew that its operating systems had to be able to access NetWare resources, but early attempts to have Novell supply a NetWare client for the Windows operating systems failed. As a result, Microsoft developed the NetWare clients for Windows, and Novell subsequently released clients of their own, which shipped with NetWare. Both have continued to update their software, and even today, you can choose between the Microsoft client for NetWare that ships with Windows or Novell's client, which you can download from their Web site.
The NetWare clients from Microsoft provided in the Windows operating systems fit into the same networking architecture as the client for Microsoft networking. To access NetWare resources in Windows 2000 Professional, you must installthe Client Service for NetWare (CSNW) and the NWLink IPX/SPX/NetBIOS Compatible Transport Protocol modules using the same procedure describedearlier in this lesson in the section entitled "Installing Windows Networking Components." In Windows 95, Windows 98, or Windows Me, the names of the modules are slightly different; you must install the Client for NetWare Networks and the IPX/SPX-compatible protocol.
The CSNW module is a second redirector that you can use along with—or instead of—the Microsoft networking client. When an application requests access to a network resource, the system determines whether the request is for a Windows or NetWare file and sends it to the appropriate redirector. The NWLink protocol module is a reverse-engineered version of Novell's IPX protocols. In most cases, Windows systems use the IPX protocols only to access NetWare servers. The NetWare redirector is connected to the NWLink protocol module, and the Microsoft redirector uses TCP/IP or NetBEUI. Both protocols' modules are then connected to the same network interface adapter driver, as shown in Figure 4.6.
Figure 4.6 Microsoft's NetWare client functions as a second redirector within the Windows networking architecture, using its own version of the IPX protocols
The CSNW included with Windows 2000 Professional and Windows NT Workstation provides basic NetWare connectivity, but Windows 2000 Server and Windows NT Server include the Gateway Service for NetWare (GSNW), which expands this functionality. In addition to providing client access to NetWare servers, GSNW also enables Windows systems without an installed NetWare client to access NetWare resources. Once you'veinstalled GSNW, the service's client capabilities enable it to connect to NetWare servers. You can then configure GSNW to share those NetWare resources using the system's Microsoft networking capabilities. When a Windows client accesses the share on the Windows NT or Windows 2000 server, the server accesses the files on the NetWare server and relays them to the client.
Novell continues to maintain its own client software packages for NetWare, which you can use instead of the ones included with the Windows operating systems. The Microsoft and Novell clients both provide the same basic functionality, such as access to NetWare volumes and printers and to NDS, but Novell's clients also provide additional capabilities that are helpful to administrators and power users.
The primary difference between the Microsoft and Novell clients is that the Novell clients include the NetWare Administrator application, which is the tool that administrators use to create and maintain objects in the NDS database. This is a critical part of NetWare administration, and it's the main reason for using Novell clients instead of Microsoft clients. Apart from including NetWare Administrator, Novell clients provide additional file management functions and utilities accessible from shortcut menus and the system tray, but they also tend to be noticeably slower than Microsoft clients.
Novell maintains the following three NetWare clients for Windows:
The Windows 95, Windows 98, Windows NT, and Windows 2000 clients all consist of modules that fit into the existing Windows networking architecture. Each client includes its own redirector—a genuine Novell IPX protocol module, rather than Microsoft's compatible version—and network interface adapter drivers that conform to the Open Data-link Interface (ODI) standard used by Novell. However, the client can use the NDIS drivers supplied with Windows, if one is already installed.
Although Novell has not added Windows Me to the list of supported clients,they claim that the client for Windows 95 and Windows 98 fully supportsWindows Me, as well.
The client for DOS and Windows 3.1 is different, because these operating systems don't have their own networking capabilities. The client provides a complete networking architecture in itself, which can also work alongside the Windows client included in Windows for Workgroups.
Macintosh computers can access network resources hosted by virtually any server operating system, but this is generally not due primarily to the capabilities of the Macintosh or the MacOS operating system. To support Macintosh clients, you nearly always have to install additional software modules on the client or a server. The Windows and NetWare server operating systems have had the ability to support AppleTalk protocols for a long time, either built in to the server operating system or through an add-on product. Windows requires you to install the Microsoft Services for Macintosh product on your Windows NT or Windows 2000 servers to support Macintosh clients. This product installs support for AppleTalk protocols on the server and makes it possible for Macintosh systems to store their files on Windows servers in their native file format. However, Microsoft Services for Macintosh does not permit Macintosh computers to share their own resources with Windows clients. The relationship between the Macintoshes and Windows machines is strictly client/server.
Macintosh computers can access NetWare servers in three ways. NetWare ships with support for AppleTalk protocols. When you install AppleTalk on a NetWare server, Macintosh clients can access the server using their built-in networking capabilities. You can also install the Novell Client for MacOS on a Macintosh computer, which provides it with support for the IPX protocols. This client is still available, but it is no longer being developed. A newer product, called NovellNative File Access for Macintosh, enables Macintosh computers to access NetWare drives using the AppleTalk Filing Protocol (AFP) over TCP/IP. Noadditional client is required on the Macintosh computer.
Macintosh computers can access UNIX systems using the standard TCP/IP communication tools that UNIX workstations use among themselves. Virtually all TCP/IP implementations include FTP and Telnet clients, and Macintosh systems can use these to access a UNIX computer just like another UNIX computer would.
Computers running UNIX or Linux are capable of functioning as clients of virtually any other operating system, but the process of configuring and connecting them is not as straightforward as it is with other clients. The Windows, NetWare, and Macintosh operating systems do not include native UNIX clients per se, but there are server capabilities built into all of these products that UNIX computers can access, and there are add-on products that provide more comprehensive client access.
Because all of the UNIX and Linux variants are based on the TCP/IP protocols, they all include the standard TCP/IP client programs, such as FTP and Telnet. This means that a UNIX client computer can connect to any system running the server versions of these applications. Some of the server operating systems also include other UNIX-compatible services. For example, Windows 2000 includes native support for the line printer remote (LPR) and line printer daemon (LPD) services, which enables Windows and UNIX computers to share printers with each other. However, to provide more complete client connectivity for UNIX computers, most of the server operating systems require the installation of an add-on product. Microsoft Windows Services for UNIX, for example, provides a Windows computer with NFS client and server capabilities, which makes it possible for Windows and UNIX computers to mount each other's file systems. The product also includes a Telnet client and server, Authentication Tools for NFS, a Remote Shell Service, and other UNIX-style utilities. Novell also has a product that provides NFS client and server capabilities, called NetWare NFS Services.