Client Server Networks

Introduction to Client Server Networks

The term client-server refers to a popular model for computer networking that utilizes client and server devices each designed for specific purposes. The client-server model can be used on the Internet as well as local area networks (LANs). Examples of client-server systems on the Internet include Web browsers and Web servers, FTP clients and servers, and DNS.
Client and Server Devices
Client/server networking grew in popularity many years ago as personal computers (PCs) became the common alternative to older mainframe computers. Client devices are typically PCs with network software applications installed that request and receive information over the network. Mobile devices as well as desktop computers can both function as clients.

A server device typically stores files and databases including more complex applications like Web sites. Server devices often feature higher-powered central processors, more memory, and larger disk drives than clients.
Client-Server Applications
The client-server model distinguishes between applications as well as devices. Network clients make requests to a server by sending messages, and servers respond to their clients by acting on each request and returning results. One server generally supports numerous clients, and multiple servers can be networked together in a pool to handle the increased processing load as the number of clients grows.

A client computer and a server computer are usually two separate devices, each customized for their designed purpose. For example, a Web client works best with a large screen display, while a Web server does not need any display at all and can be located anywhere in the world. However, in some cases a given device can function both as a client and a server for the same application. Likewise, a device that is a server for one application can simultaneously act as a client to other servers, for different applications.

[Some of the most popular applications on the Internet follow the client-server model including email, FTP and Web services. Each of these clients features a user interface (either graphic- or text-based) and a client application that allows the user to connect to servers. In the case of email and FTP, users enter a computer name (or sometimes an IP address) into the interface to set up connections to the server.
Local Client-Server Networks
Many home networks utilize client-server systems without even realizing it. Broadband routers, for example, contain DHCP servers that provide IP addresses to the home computers (DHCP clients). Other types of network servers found in home include print servers and backup servers.
Client-Server vs Peer-to-Peer and Other Models
The client-server model was originally developed to allow more users to share access to database applications. Compared to the mainframe approach, client-server offers improved scalability because connections can be made as needed rather than being fixed. The client-server model also supports modular applications that can make the job of creating software easier. In so-called "two-tier" and "three-tier" types of client-server systems, software applications are separated into modular pieces, and each piece is installed on clients or servers specialized for that subsystem.

Client-server is just one approach to managing network applications The primary alternative, peer-to-peer networking, models all devices as having equivalent capability rather than specialized client or server roles. Compared to client-server, peer to peer networks offer some advantages such as more flexibility in growing the system to handle large number of clients. Client-server networks generally offer advantages in keeping data secure.

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What Is the IP Address of a Router?

A typical home network router possesses two IP addresses, one for the internal home (LAN) and one for the external Internet (WAN) connection. How can you find the router IP addresses?

he internal, LAN-IP address is normally set to a default, private number. Linksys routers, for example, use 192.168.1.1 for their internal IP address. D-Link and Netgear routers typically use 192.168.0.1. Some US Robotics routers use 192.168.123.254, and some SMC routers use 192.168.2.1. No matter the brand of router, its default internal IP address should be provided in documentation. Administrators often have the option to change this IP address during router setup. In any case, however, the private LAN-IP address remains fixed once set. It can be viewed from the router's administrative console.

The external, WAN-IP address of the router is set when the router connects to the Internet service provider. This address can also be viewed on the router's administrative console. Alternatively, the WAN-IP address can be found by visiting a Web-based IP address lookup service like www.checkip.dyndns.org/ from any computer on the home LAN.

Another way to identify the public IP addresses of routers, involves executing a ping or "traceroute" command. From inside a home network, the (DOS) command "ping -r 1" will send a message through the home router that will cause its IP address to be displayed. For example, "ping -r 1 www.yahoo.com" should result in a message like the following displayed on the command prompt:

Reply from 66.94.230.43: bytes=32 time=294ms TTL=56
Route: 209.179.21.76
...

In this example, the IP address after "Route:" (209.179.21.76) corresponds to the router WAN address.



On corporate networks, network discovery services based on SNMP can automatically determine the IP addresses of routers and many other network devices.

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The ISO/OSI Reference Model

The International Standards Organization (ISO) Open Systems Interconnect (OSI) Reference Model defines seven layers of communications types, and the interfaces among them. (See Figure 1.) Each layer depends on the services provided by the layer below it, all the way down to the physical network hardware, such as the computer's network interface card, and the wires that connect the cards together.

An easy way to look at this is to compare this model with something we use daily: the telephone. In order for you and I to talk when we're out of earshot, we need a device like a telephone. (In the ISO/OSI model, this is at the application layer.) The telephones, of course, are useless unless they have the ability to translate the sound into electronic pulses that can be transferred over wire and back again. (These functions are provided in layers below the application layer.) Finally, we get down to the physical connection: both must be plugged into an outlet that is connected to a switch that's part of the telephone system's network of switches.

If I place a call to you, I pick up the receiver, and dial your number. This number specifies which central office to which to send my request, and then which phone from that central office to ring. Once you answer the phone, we begin talking, and our session has begun. Conceptually, computer networks function exactly the same way.

It isn't important for you to memorize the ISO/OSI Reference Model's layers; but it's useful to know that they exist, and that each layer cannot work without the services provided by the layer below it.

Figure 1: The ISO/OSI Reference Model

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What is a Network?

A ``network'' has been defined[1] as ``any set of interlinking lines resembling a net, a network of roads || an interconnected system, a network of alliances.'' This definition suits our purpose well: a computer network is simply a system of interconnected computers. How they're connected is irrelevant, and as we'll soon see, there are a number of ways to do this.

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Introduction to Networking

A basic understanding of computer networks is requisite in order to understand the principles of network security. In this section, we'll cover some of the foundations of computer networking, then move on to an overview of some popular networks. Following that, we'll take a more in-depth look at TCP/IP, the network protocol suite that is used to run the Internet and many intranets.

Once we've covered this, we'll go back and discuss some of the threats that managers and administrators of computer networks need to confront, and then some tools that can be used to reduce the exposure to the risks of network computing.

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Wireless Access Point

A Wireless Access Point (WAP) is a network device which is configured on WLANs (Wireless Local Area Networks) using an IP address to enable wireless communication usually between any communication devices (wired or wireless) and particularly between wireless devices and wired networks. In a corporate network environment, a WAP can be attached to a wired network and configured to enable wireless access to another LAN in a different location. A WAP typically acts as a gateway for clients to access a wired network by enabling reception and transmission of WLAN signals.

In a way, a WAP makes possible wireless communication with mobility within its technically-specified range. The WAPs can also be used effectively to wirelessly connect different wired LANs within a specified range and particularly where the wired connectivity is not normally feasible. Standard WAPs can support up to 255 clients.

A Wireless Access Point (WAP) is a device that receives signals from wireless devices which are within reach and from there, allows them to gain access to the Internet or a network. A WAP can be viewed as a hub, but it can also act as a bridge to increase the range of wireless networks.

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Introduction to Network Types

LAN, WAN and Other Area Networks

One way to categorize the different types of computer network designs is by their scope or scale. For historical reasons, the networking industry refers to nearly every type of design as some kind of area network. Common examples of area network types are:

• LAN - Local Area Network
• WLAN - Wireless Local Area Network
• WAN - Wide Area Network
• MAN - Metropolitan Area Network
• SAN - Storage Area Network, System Area Network, Server Area Network, or sometimes Small Area Network
• CAN - Campus Area Network, Controller Area Network, or sometimes Cluster Area

• Network
• PAN - Personal Area Network
• DAN - Desk Area Network

LAN and WAN were the original categories of area networks, while the others have gradually emerged over many years of technology evolution.

Note that these network types are a separate concept from network topologies such as bus, ring and star.

LAN - Local Area Network

A LAN connects network devices over a relatively short distance. A networked office building, school, or home usually contains a single LAN, though sometimes one building will contain a few small LANs (perhaps one per room), and occasionally a LAN will span a group of nearby buildings. In TCP/IP networking, a LAN is often but not always implemented as a single IP subnet.

In addition to operating in a limited space, LANs are also typically owned, controlled, and managed by a single person or organization. They also tend to use certain connectivity technologies, primarily Ethernet and Token Ring.


WAN - Wide Area Network

As the term implies, a WAN spans a large physical distance. The Internet is the largest WAN, spanning the Earth.

A WAN is a geographically-dispersed collection of LANs. A network device called a router connects LANs to a WAN. In IP networking, the router maintains both a LAN address and a WAN address.

A WAN differs from a LAN in several important ways. Most WANs (like the Internet) are not owned by any one organization but rather exist under collective or distributed ownership and management. WANs tend to use technology like ATM, Frame Relay and X.25 for connectivity over the longer distances.

LAN, WAN and Home Networking

Residences typically employ one LAN and connect to the Internet WAN via an Internet Service Provider (ISP) using a broadband modem. The ISP provides a WAN IP address to the modem, and all of the computers on the home network use LAN (so-called private) IP addresses. All computers on the home LAN can communicate directly with each other but must go through a central gateway, typically a broadband router, to reach the ISP.
Other Types of Area Networks
While LAN and WAN are by far the most popular network types mentioned, you may also commonly see references to these others:

• Wireless Local Area Network - a LAN based on WiFi wireless network technology
• Metropolitan Area Network - a network spanning a physical area larger than a LAN but smaller than a WAN, such as a city. A MAN is typically owned an operated by a single entity such as a government body or large corporation.
• Campus Area Network - a network spanning multiple LANs but smaller than a MAN, such as on a university or local business campus.
• Storage Area Network - connects servers to data storage devices through a technology like Fibre Channel.
• System Area Network - links high-performance computers with high-speed connections in a cluster configuration. Also known as Cluster Area Network.

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SSID - (Service Set Identifier)

Definition: An SSID is the name of a wireless local area network (WLAN). All wireless devices on a WLAN must employ the same SSID in order to communicate with each other.

The SSID on wireless clients can be set either manually, by entering the SSID into the client network settings, or automatically, by leaving the SSID unspecified or blank. A network administrator often uses a public SSID, that is set on the access point and broadcast to all wireless devices in range. Some newer wireless access points disable the automatic SSID broadcast feature in an attempt to improve network security.

SSIDs are case sensitive text strings. The SSID is a sequence of alphanumeric characters (letters or numbers). SSIDs have a maximum length of 32 characters.
Also Known As: Service Set Identifier, Network Name
Examples: Wardrivers sometimes scan for the SSIDs being broadcast by wireless LANs, then set that SSID on their client to attempt to join that WLAN. Knowing the SSID name does not necessarily mean that rogue clients will be able to join the network. It depends on how the network administrator has configured their WLAN, particularly WEP security.

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What is (Wireless / Computer) Networking?

In the world of computers, Networking is the practice of linking two or more computing devices together for the purpose of sharing data. Networks are built with a mix of computer hardware and computer software.

What Is a Network Name?

Did you know a network name is a text string that devices use to reference a particular computer network? These strings are, strictly speaking, separate from the names of individual network devices and the addresses they use to identify each other. However, several different forms of network naming exist and it's fairly common (even in the IT world) for people to blur the distinction between computer and network names in casual conversation.


SSID

Wi-Fi networks support a type of network name called SSID. Wi-Fi access points and clients are each always assigned an SSID to help identify each other. When a person speaks of wireless network names, they typically are referring to SSIDs.

Definition: An SSID is the name of a wireless local area network (WLAN). All wireless devices on a WLAN must employ the same SSID in order to communicate with each other.

The SSID on wireless clients can be set either manually, by entering the SSID into the client network settings, or automatically, by leaving the SSID unspecified or blank. A network administrator often uses a public SSID, that is set on the access point and broadcast to all wireless devices in range. Some newer wireless access points disable the automatic SSID broadcast feature in an attempt to improve network security.

SSIDs are case sensitive text strings. The SSID is a sequence of alphanumeric characters (letters or numbers). SSIDs have a maximum length of 32 characters.
Also Known As: Service Set Identifier, Network Name
Examples: Wardrivers sometimes scan for the SSIDs being broadcast by wireless LANs, then set that SSID on their client to attempt to join that WLAN. Knowing the SSID name does not necessarily mean that rogue clients will be able to join the network. It depends on how the network administrator has configured their WLAN, particularly WEP security.

Change the Default SSID on Wireless Access Points and Routers

Wi-Fi access points and routers ship with a pre-defined network name (SSID) set by the manufacturer.

The SSID can be accessed from within these products' Web-based or Windows-based configuration utilities. Common examples of pre-defined SSIDs are simple names like "wireless," "netgear," "linksys," or "default." An SSID can be changed at any time, as long as the change is also made on all wireless clients.

To improve the security of your home wireless network, change the SSID to a different name than the default. Here are some recommended do's and dont's, based on best network security practices:

• Don't use your name, address, birthdate, or other personal information as part of the SSID.
• Likewise, don't use any of your Windows or Internet Web site passwords.
• Don't tempt would-be intruders by using tantalizing network names like "SEXY-BOX" or "TOP-SECRET".
• Do pick an SSID that contains both letters and numbers
• Do choose a name as long or nearly as long as the maximum length allowed.
• Do consider changing your SSID every few months.

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