Although every device on a LAN is connected to every other device, they do not necessarily communicate with each other. There are two basic types of LANs, based on the communication patterns between the machines: client/server networks and peer-to-peer networks.
On a client/server network, every computer has a distinct role: that of either a client or a server. A server is designed to share its resources among the client computers on the network. Typically, servers are located in secured areas, such as locked closets or data centers (server rooms), because they hold an organization’s most valuable data and do not have to be accessed by operators on a continuous basis. The rest of the computers on the network function as clients (see image below).
A dedicated server computer often has faster processors, more memory, and more storage space than a client because it might have to service dozens or even hundreds of users at the same time. High-performance servers typically use from two to eight processors (and that’s not counting multi-core CPUs), have many gigabytes of memory installed, and have one or more server-optimized network interface cards (NICs), RAID (Redundant Array of Independent Drives) storage consisting of multiple drives, and redundant power supplies. Servers often run a special network OS—such as Windows Server, Linux, or UNIX—that is designed solely to facilitate the sharing of its resources. These resources can reside on a single server or on a group of servers. When more than one server is used, each server can “specialize” in a particular task (file server, print server, fax server, email server, and so on) or provide redundancy (duplicate servers) in case of server failure. For demanding computing tasks, several servers can act as a single unit through the use of parallel processing.
A client computer typically communicates only with servers, not with other clients. A client system is a standard PC that is running an OS such as Windows. Current OSes contain client software that enables the client computers to access the resources that servers share. Older OSes, such as Windows 3.x and DOS, required add-on network client software to join a network.
By contrast, on a peer-to-peer network, every computer is equal and can communicate with any other computer on the network to which it has been granted access rights. Essentially, every computer on a peer-to-peer network can function as both a server and a client; any computer on a peer-to-peer network is considered a server if it shares a printer, a folder, a drive, or some other resource with the rest of the network. This is why you might hear about client and server activities, even when the discussion is about a peer-to-peer network.
Peer-to-peer networks can be as small as two computers or as large as hundreds of systems and devices. Although there is no theoretical limit to the size of a peer-to-peer network, performance, security, and access become a major headache on peer-based networks as the number of computers increases. In addition, Microsoft imposes a limit of only 5, 10 or 20 concurrent client connections to computers running Windows. This means that a maximum of 20 (or fewer) systems will be able to concurrently access shared files or printers on a given system. This limit is expressed as the “Maximum Logged On Users” and can be seen by issuing the NET CONFIG SERVER command at a command prompt. This limit is normally unchangeable and is fixed in the specific version and edition of Windows as follows:
- 5 users: Windows XP Home, Vista Starter/Home Basic
- 10 users: Windows NT, 2000, XP Professional, Vista Home Premium/Business/Enterprise/Ultimate
- 20 users: Windows 7 (all editions)
When more than the allowed limit of users or systems try to connect, the connection is denied and the client sees one of the following error messages:
Operating system error 71. No more connections can be made to this remote computer at this time because there are already as many connections as the computer can accept.
System error 71 has occurred. This remote computer has reached its connection limit, you cannot connect at this time.
Even though it is called a “Server” OS, Windows Home Server also has the same 10-connection limit as the non-Home client Windows versions of XP and Vista. If you need a server that can handle more than 10 or 20 clients, I recommend using a Linux-based server OS (such as Ubuntu Server) or one of the professional Windows server products (such as Windows 2000 Server, Server 2003, Server 2008, Essential Business Server, or Small Business Server). Peer-to-peer networks are more common in small offices or within a single department of a larger organization. The advantage of a peer-to-peer network is that you don’t have to dedicate a computer to function as a file server. Instead, every computer can share its resources with any other. The potential disadvantages to a peer-to-peer network are that typically less security and less control exist because users normally administer their own systems, whereas client/server networks have the advantage of centralized administration.
Note that the actual networking hardware (interface cards, cables, and so on) is the same in client/server versus peer-to-peer networks, it is only the logical organization, management and control of the network that varies.
Comparing Client/Server and Peer-to-Peer Networks
Client/server LANs offer enhanced security for shared resources, greater performance, increased backup efficiency for network-based data, and the potential for the use of redundant power supplies and RAID drive arrays. Client/server LANs also are more expensive to purchase and maintain. The following table compares client/server and peer-to-peer server networking.
|Comparing Client/Server and Peer-to-Peer Networking|
|Access control||Via user/group lists of permissions Via user/group lists of permissions to only the resources granted, and different users can be given different levels of access.||Resources are managed by each system with shared resources. Depending on the OS, resources may becontrolled by separate passwords for each shared resource or by a user list stored on each system with shared resources. Some OSs do not use passwords or user/group lists, thus enabling access to shared resources for anyone accessing the network.|
|Security||High; access is controlled by user or by group identity.||Varies; if password protection is employed, anyone who knows the password can access a shared resource. If no passwords are used, anyone who can access the workgroup can access shared resources. However, if user/group names are used,security is comparable to a client/server network.|
|Performance||High; the server is dedicated and doesn’t handle other tasks.||Low; servers often act as workstations.|
|Hardware Cost||High; specialized high-performance server hardware with redundancy features.||Low; any workstation can become a server by sharing resources.|
|Software Cost||Higher; license fees per user are part of the cost of the server OS.||Lower; client software is included with OS.|
|Backup||Centralized on the server; managed by network administrator. Backup by device and media only required at server.||Decentralized; managed by users. Backup devices and media are required at each workstation.|
|Redundancy||Yes; duplicate power supplies, hot-swappable drive arrays, and even redundant servers are common; network OS normally is capable of using redundant devices automatically.||No true redundancy among peer “servers” or clients; failures require manual intervention to correct, with a high possibility of data loss.|