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Tuesday, November 24, 2009

How to Compute an IP Subnet Mask

IP address is a code that has host and network parts. The host bits describe a particular PC. The network prefix shows a network; its measurement lengthwise depends on the class of network. Sub-netting helps to arrange a network by splitting it to several subnets. To describe such subnets, you should take bits from the portion of host of the IP address. That also expands the network prefix. The subnet mask openly describes network and host bits as 1/0, respectively.

In this instance, we will compute a subnet mask for a PC with IP address “ that goes to network with 6 subnets.

1. Step 1

Verify the network class A/B/C based on IP addresses:

* If the IP addresses start with 1 - 126, it is Class A.
* If the IP addresses start with 128 - 191, it is Class B.
* If the IP addresses start with 192 - 223, it is Class C.

In our illustration, the network is class C because the IP address begin with 192.

1. Step 2

Know the number of bits required to define subnets:

* Number of subnets = (2^ # of bits) - 2. Therefore,
* Number of bits = Log2(# of subnets + 2).

In our illustration, there are 6 subnets:

* Number of bits = Log2(6 + 2) = Log2(8) = 3. 3 bits in the IP address are utilized as a subnet portion.

1. Step 3

Create the subnet mask in a binary form by expanding the original subnet mask w/ subnet bits. Original subnet mask for class A - C are:

* 11111111.00000000.00000000.00000000 (Class A, network part = 8 bits)
* 11111111.11111111.00000000.00000000 (Class B, network part = 16 bits)
* 11111111.11111111.11111111.00000000 (Class C, network part = 24 bits)

In our illustration, an addition of the default “class C” subnet mask with three bits (Step 2) outcomes in the subnet mask

1. Step 4

Change the subnet mask binary to the decimal form. The binary form has 4 octets. Utilize the following rules:

* Write 255 for "1111111" octet.
* Write 0 for "00000000" octet.
* If octet has 1 and 0 utilize the formula:

Integer # is equals to (128 x n) + (64 x n) + (32 x n) + (16 x n) + (8 x n) + (4 x n) + (2 x n) + (1 x n)
Where N is “1” or “0” in the corresponding place in the sequence of octet.

In our illustration, for 11111111.11111111.11111111.11100000
11111111 = 255
11111111 = 255
11111111 = 255
11100000 = (128 x 1) + (64 x 1) + (32 x 1) + (16 x 0) + (8 x 0) + (4 x 0) + (2 x 0) + (1 x 0) = 224

The Subnet mask is “”.[]

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Monday, November 23, 2009

What is SAN or Storage Area Network?

A SAN or Storage Area Network
is a network that is separate from WANs and LANs. It is used to link all the storage resources linked to different servers. It consist a collection of SAN Software and SAN Hardware; the hardware has high interconnection rates among the different storage devices and the software configures the SAN and manages monitors.

Storage Area Network or SAN is a high speed sub-network of shared storage devices. A storage device is a machine that has nothing but disks for saving data. A Storage Area Network's architecture acts in a way that makes every storage devices accessible to every server on a WAN or LAN. As other storage devices are placed to a Storage Area Network, they too will be available from any server in the bigger network. In this case, the server only acts as a way between the user and the stored file or data.

A Storage Area Network is a storage devices network that is linked to each other and to a server, in some arrangements a Storage Area Network is also linked to the network. It is foretell to become the storage data technology of selection in the coming years.

Storage Area Networks originated to conquer the problems with NAS or network attached storage devices, which like ordinary servers are tough to handle and hard to expand the capability on. Network Attached Storage devices also put in to the traffic on the network and endure from the delays made by the OS’s network stacks.

A Storage Area Network is made of a lot of fabric switches linked in a network. The most known form of Storage Area Network utilizes the “Fiber Channel fabric protocol”. On the other hand, ISCSI can be utilized with IP switches. The Storage Area Network enables the storage on the hard drive in the Disk collection controllers to be connected among the servers.

One of the major benefits of Storage Area Network is that it makes things easier for the network infrastructure and simplifies to manage. This it does by means of virtualization, consolidation, integration, and automation. Consolidation targets at centralizing the storage to enhance scalability, lessening infrastructure complexity, and raising efficiency. Virtualization aids to enhance availability and lessens costs as it gives a wide view of storage devices. Automation of habit tasks enables the administrators to concentrate on vital tasks. Automation also enhances responsiveness. Integration aids organizations furnish users with the wanted info in a more systematic manner.[]

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Saturday, November 14, 2009

5 Minutes to Understanding Tcp/ip

What exactly is TCP/IP?

TCP/IP stands for Transmission Control Protocol/Internet Protocol. It is a software language and set of rules computers and/or devices use on a network to talk to each-other. TCP/IP is both a LAN (local area network) and WAN (wide area network) protocol. A WAN spans a great distance. The internet
is the biggest WAN in existence. A protocol is a set of rules as well as a software language used in order to transmit services, identify information, and identify computers. TCP/IP is a very scalable (able to grow and support new or different services and computers), robust (reliable) protocol. This is mainly why it was elected a long time ago to be the default internet protocol. In order for two computers to exist and effectively communicate they must either:

A. Both be using the same protocol


B. Use an intermediary program to interface and act as an interpreter between the two different protocols.

In this situation example A. is the most ideal and common when speaking of computers existing and effectively communicating on the internet. If you are speaking "english" and I am speaking "german" you would not be able to understand me, correct? Computers are the same way. The language and set of rules that computers and devices use to speak and identify each-other on the internet guessed is TCP/IP. Right now we use the version known as TCP/IP v4. The v4 stands for "version 4". We are starting to ever so slowly move to TCP/IP version 6. At the time this article is written (January, 2008) TCP/IP v6 is certainly not common place. However in the future we will inevitably all need to be able to use it as it becomes the default TCP/IP protocol.

Where is TCP/IP exactly?

This software is, by default, installed in almost every operating system used today. It is already there within your Windows, Macintosh, or Linux operating system when you start your computer. Programmers have it already pre-installed and integrated with your operating system right out of the box.

Can you uninstall TCP/IP?

Yes, in most operating systems you can. However situations in why you would do such is beyond the scope of this article and you, as the general computer user would almost never (I know, "never say never". OK 99.999 % of the time the common computer user would never need to do such).

In order to use TCP/IP on the internet you need three common parameters. They look like this:

IP Address:

Default Gateway Address:

Subnet Mask:

These parameters or TCP/IP settings can be accessed or viewed in your operating system. Every operating system will have at least one place where you can see and if needed change these IP settings.NOTE: If you are a new user and are not having any connectivity issues I would leave these settings as is for now.

What is an IP Address?

When using the TCP/IP protocol a machine, and or website needs an identifier. This identifier is an "Ip Address". It is actually 32 ones and zero's. However using simple decimal shorthand you can break this down to normal "decimal" numbers which is what we generally see. Hence an IP address is four separate decimal numbers separated by an period. IP Addresses look like this:

This number identifies your machine. This way other computers know where to find you and/or who to respond to on the internet when you request any service or want to browse around online. In order for computers on the internet not to get confused, no two machines, or websites can have the same IP address. They all must be "unique" to the machine or website.

What is a Default Gateway Address?

A "default gateway" IP address is needed as well. this is the IP address of the machine you use to get onto the internet through. It is a doorway so to speak. Every internet based machine goes out to the internet through a "default gateway". This is normally a server or a router.

A default gateway machine also has an IP address. It is known as the "gateway address" or gateway IP.

Your computer as an internet machine also needs this IP configured.

What is the Subnet Mask?

A subnet mask , to put it as simple as possible is a indicator address that does two things.

1. identifies your internet service provider network and this networks size. 2. Helps machines narrow in on your exact location when sending information to you but this is out of the scope of this article. For now just understand it's basis and understand you need one in order to access and exist on the internet.

So now you know what an IP address, a gateway address, and what a subnet mask address is. These are the internet settings (aka IP Parameters) every machine needs to access the internet.

So how and where do you get them? Well you can manually configure your own every time you log in or start your computer. You should contact your internet service provider to ask what your assigned IP, default gateway address, and subnet mask should be. However this would be a pain in the neck wouldn't it?

This is where DHCP (dynamic host configuration protocol) comes in. We will discuss this in the near future. []

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Thursday, November 12, 2009


This address is utilized by Linksys routers and open source linux based firm wares that are well matched with Linksys routers.As a result is apt to be set as default gateway on PCs connected to a router. as well commonly includes web interface which controls the configuration of the router. If you are can’t access this address it is likely that you need some kind of network configuration issue.

The IP address is the default for the Linksys brand-home for broadband routers. The manufacturer at the factory sets this IP, but you could change it any time utilizing the network administrative console of the router.

IP address is a private IP v4 network address. Any brand of the router’s network, or any PC on a limited network for that issue, could be set to utilize this address. As with any other Internet Protocol address, however, only a single device on the network must utilize to prevent address conflicts. is a Class C private Internet Protocol address. Private Internet protocol addresses are preserved for private networks which are not included on the Internet. Though most home networks are linked to internet, they’re not part of it. Instead, the network router operates as the gateway and routes network data to correct PC on the home network. While this permits a lot of PCs to share single Internet Service Provider connection the disadvantage is that home PCs are not reachable right away from the Internet (but given the number of trojans and viruses out there this could also be a great thing).[]

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Tuesday, November 10, 2009

How to Implement Wireless VLANS

The wireless access points operate as bridges with no routing defined anywhere on the wireless network segment. All VLANs are defined on the wired switches and mapped with specific SSIDs at each access point. The maximum number of VLANs and SSIDs per access point that can be mapped is 16. The wireless client attaches or associates with a specific SSID which in turn will map client with membership in a specific VLAN.

There is an option to configure the maximum number of wireless client associations allowed per SSID improving network performance and availability. The access point is assigned a primary SSID with the 802.11 standard, advertising it with beacons on that segment to all wireless clients. There is a guest SSID defined that companies should define a VLAN policy for that group or with access control list security policies denying access to the corporate network. Guest traffic for the most part should be directed across the internet unless they have specific network rights.

VLAN membership of each wireless client is assigned considering what servers are most accessed, specific company department and security rights. Device types such as a scanner with less security won't be assigned the same VLAN as an engineering group with sensitive information and 802.1x security.

VLAN 1 is the default native VLAN and doesn't tag traffic. The native VLAN number assigned on the wired switches must match the VLAN assigned at all attached access points on that network segment. The native VLAN is sometimes assigned to network management traffic or the RADIUS server. Companies will implement access control lists at each network switch to filter traffic securing the management VLAN traffic. With most designs the native VLAN isn't mapped to a SSID except with connecting root bridges and non root bridges. Define an infrastructure SSID for infrastructure devices such as a repeater or workgroup hub and map the native VLAN allowing those devices to associate with non root bridge and root bridges.

Wireless clients configured with 802.1x authentication will have a RADIUS server configured with mapped SSIDs per wireless client. This is called RADIUS SSID control. The server sends the list to the access point where the client is allowed to associate with an access point should they be a member of one or several SSIDs. RADIUS VLAN control assigns each client with a specific VLAN and default SSID. The mapping can be overridden with the RADIUS sever configuration. During authentication the wireless client is assigned to that specific VLAN. The employee however can't be a member of any wired VLAN except that. Policy group filters or class map policies can be defined per VLAN. You should deny all infrastructure devices to be members of any non-infrastructure SSID. Wireless clients will see all broadcasts and multicasts of all mapped VLANs unless 802.1x per VLAN encryption is implemented with TKIP, MIC and broadcast keys.

Trunking is implemented to switch traffic between network segments that have multiple VLANs defined. Each VLAN defines a separate broadcast domain comprised of a group of employees with a company department. The trunk is a physical switch port interface with defined Ethernet subinterfaces configured with 802.1q or ISL encapsulation. Those packets are tagged with specific VLAN number before it is sent between access point and wired network switch. The access point Ethernet interface is configured as a hybrid trunk. Access control lists should be defined at the wired switch Ethernet interface that drops packets from VLANs not defined with any SSID.

VLAN 100 = 192.168.37.x - SSID = Engineers

VLAN 200 = 192.168.38.x - SSID = Guest

VLAN 300 = 192.168.39.x - SSID = Sales

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Thursday, October 29, 2009

Basics of Broadband Internet

Involving high-speed broadband Internet using a modem. This allows you to access information on the Internet very quickly, so often referred to as a high-speed Internet. Broadband Internet can be in the form of DSL, cable, or satellite. As the popularity of the Internet exploded in the early 2000's, Broadband Internet emerged, offering computer users a better thing. It's quite expensive at first, but has become more affordable. Today is a feature throughout the world.

If you are still using dial-up internet connection then you may be wondering what all the hype is about. However, it does not make you connect to the internet, too! Most people switch their home for Broadband Internet Service after experiencing the difference between the two while accessing the Internet on their office computers. You can go to a public library is also to try a computer with Broadband Internet for several hours, and then decide which one you prefer.

DSL is the most common type of Broadband Internet connections. DSL stands for Digital Subscriber Line. This allows a high volume of data to be sent quickly. Some rural areas have to offer cable Broadband connection. One downside of it is that all customers share a certain bandwidth, so you can find your Broadband Internet functions slowly at times. Cable companies try to avoid this by adding additional connections as the number of subscribers increases. A satellite Broadband connection works the same way.

Broadband Internet has become very popular because it offers so many advantages over dial-up Internet access. It's up to 10 times faster. This means you can access information on the Internet almost instant you click on it. Because not operate on your phone line, you can still receive phone calls while connected to the internet. This also means you will not be interrupted in the middle of something or not will be online during peak hours.

Many people choose to watch movies and videos on the internet. After Broadband Internet allows you to download them or see them quickly like that does not take long for their support. Listening and downloading music is another popular feature on the Internet that works best with Broadband Internet services.

Internet broadband can be purchased from various providers. This has become a popular feature Internet service providers and cable providers. This is a great opportunity for you to get cable service package agreement and Broadband Internet. You'll save money over having them in the two providers. Broadband Internet is more expensive than dial-up Internet service, but well worth the speed that you get to access the Internet./ Source :

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Wednesday, October 28, 2009

The Work Of Wireless Internet

The advantage of wireless internet

Wireless internet has many advantages both for business and residential users. Provide broadband-speed wireless Internet is very good. Today, at over 2 Mbps for commercial use (megabytes per second) obtained using the wireless Internet.

Previously up to 1 Mbps speeds are acceptable for wireless internet, but with content providers who provide speed at 8 Kbps and up to business and residential customers through cable (fiber optic), wireless internet expectations running too high. This could mean only one thing-speed wireless internet sooner or later it will catch up with the speed offered through fiber optic cable.

Wireless Internet is more reliable than the Internet through a cable or satellite. Initial cost to the service provider will also be reduced because they do not have to spend paying expensive cable or satellite transmissions are for. All that is needed is to establish internet tower (very similar to the cellular tower). Users will be connected to the internet for his laptop / device is receiving signals from the nearest tower (once again, very similar to using a mobile phone).

Comparison between the cellular networks work and the wireless Internet network can be made to a certain extent, but working from a wireless Internet network is much more complex in nature. So, how wireless Internet works?

How WiMAX works
We can single out WiMAX (Worldwide Interoperability for Microwave Access) technology is widely used for setting up wireless internet. High speed can be achieved through a wireless network using this technology. Based on the IEEE 802.16 communications standard, this technology fast replacing complicated if cable and satellite networks expensive.

WiMax network consisting of base stations (similar to cellular tower) is set by the service provider, and a receiver antenna (similar to mobile phone antenna) at the user end. Users pay for the service provider wireless internet access, just as they would for a normal internet connection via cable networks. The service provider will provide end users with software, a login and password. Most manufacturers today complements the high-end laptop models with built in antenna is bundled with the software needed to be WiMax compatible units. Internet service providers semaphore from the base station. Antenna at the user end to catch the signal, which provides for uninterrupted internet signal is available. With a laptop equipped with an antenna you can connect to the Internet wherever a signal is available from the base station. As with cellular phone signals from nearby towers from certain service providers, as well as a new generation of WiMax services. One WiMAX base station can send signals over distances several miles depending on terrain. The flatter the terrain, more coverage. Once you move from one base station to another, the recipient will connect your laptop to another base station (from the same service provider) with a stronger signal. For WiMAX service providers to spread their operations, everyone will connect with truly high speed internet 24x7 wherever they are.

Another common use of wireless internet
Wireless Internet is not limited to WiMax. Most of us have used a wireless internet some time or another. GPRS (General Packet Radio Service) is one example. When a mobile user subscribed to a GPRS service, the recipient mobile unit can receive voice and data signals. Simply put, mobile phones can be used as a phone for voice calls and also as a modem to connect to a computer to surf the internet. The only limiting factor here is a very slow pace. This technology was gradually replaced by EDGE (Enhanced Data Rates for Global Evolution) technology. EDGE is an enhancement over GPRS, able to provide greater capacity for voice traffic and at the same time have a high speed data transfer capabilities.

EDGE technology has recently been replaced by 3G (Third Generation) technology. Speeds of 2Mbps is achieved by using 3G technology. This speed will continue to increase with technological advances. 4G (Fourth Generation) will be the next major change in this domain. 4G research is being done, but nothing that has not been allocated frequencies for this technology. It is estimated that 4G will be around 2010-2012, but with the speed of technological advance, it could happen earlier.

Mobile phones that have passed the stage where they used to just make and receive calls. Now people can buy a mobile phone to do almost anything that your personal computer or laptop can be. The operating system can be loaded into the phone, almost making them a tool for all your phone and computing needs. This is where the wireless Internet will play an important role in the future. This will connect you 24x7 with almost no damage.

Cable still be used for the Internet to wireless internet time to adjust the speed of data transfer cable. Once this happens, technologies such as WiMax and 3G will help in making this world a 24x7 connected, wire-free world./source :

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Wednesday, October 21, 2009


Internet bandwidth (the speed at which your computer can send and receive information) is measured in either Kbps (kilobits per second) or Mbps (megabits per second). If you are lucky enough to have multiple broadband options in your area, compare these factors:

Downstream bandwidth. This is the speed with which your computer can receive information from the Internet. The higher the downstream bandwidth, the faster your computer can display Web pages, transfer music, and download files. For most people, downstream bandwidth is more important than upstream bandwidth, so the speeds tend to be much higher. For example, a cable modem service might offer 6,000 Kbps downstream and only 768 Kbps upstream.

Upstream bandwidth. This is the amount of data your computer can send to the Internet. This isn't important if you just plan to read e-mail and surf the Web, because your computer only needs to send a small request in order to receive a large Web page or e-mail. However, if you're into online gaming or you want to send large files to people, then higher upstream bandwidth is important, and you should choose the highest upstream bandwidth available.

Reliability and customer service. ISP reliability has increased significantly in recent years; however, it is still not as reliable as your phone or television service. There is no objective way to measure reliability and customer service, so you should talk to your neighbors about their experiences and search the Web for reviews of ISPs in your area.
Top of pageTop of page

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Tuesday, October 20, 2009

Internet service provider (ISP)

The first step in connecting to the Internet is to find an Internet service provider (ISP). The most important services an ISP offers are:

Internet access. Access any Web site, send instant messages to your friends, play online games, or use any other Internet service.
E-mail. You can access your e-mail with Microsoft Outlook Express or your Web browser. Most ISPs offer multiple e-mail addresses, so everyone in your family can have an account. ISPs typically provide spam filtering that reduces, but does not eliminate, unwanted messages.

Depending on your location, you might have several different choices for Internet access. Starting with the most attractive technologies for home Internet access, common Internet connection types are:

Cable modems. The best performing and most affordable option available to customers, most cable TV providers offer broadband Internet access.
DSL. An excellent choice for businesses, DSL typically offers better reliability than cable modems. However, DSL tends to be more expensive than cable modems for similar levels of service.
Dial-up. The slowest method of connecting to the Internet, dial-up enables you to connect to the Internet using your existing phone lines. Dial-up is convenient because it is available to any location with a phone. However, slow performance makes using the Internet frustrating.
Satellite. Satellite broadband services provide high-speed Internet access to any location with a clear view of the sky (currently available in North America and certain other locations). Satellite services may be the only broadband option for people living in rural areas. The cost of satellite services is significantly higher than other services. While you can transfer large files quickly with satellite, browsing the Web or playing online games can seem slower than with dial-up because of the delay caused by sending signals to and from satellites.

Additionally, ISPs are beginning to offer wireless or fiber broadband Internet access in limited areas.

To find an ISP, you should contact your cable television provider for cable modem service or your telephone company for DSL. Almost all cable and telephone companies offer broadband Internet access, and they typically offer a discount if you purchase multiple services from them.

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Saturday, October 3, 2009

Common physical components of a network

There are 4 major categories of physical components in a computer network:
1. PCs
2. Interconnections: they consist of components
that provide means for data to travel
from one point to another point in the network, this category includes components
such as the following:

1. NICs: (Network Interface Cards), translates the data produced by the computer
into a format that can be transmitted over the local network (digital to
2. Network media: such as cables or wireless media, that provide the means by
which the signals are transmitted from one network device to another.
3. Connectors: provide the connection points for the media, i.e RJ-45
3. Switches: devices that provide network attachments to the end systems and
intelligent switching of the data within the local network.
4. Routers: interconnects networks and chooses the best path between networks for data
to go through.
Source :

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Friday, October 2, 2009

Logical Topologies

The Logical topology defines how the systems communicate across the physical topologies. In CISSP terms, you may hear logical topology referred to as the LAN media access method or network access method. There are two main types of logical topologies:

shared media topology
token-based topology

Shared Media

In a shared media topology, all the systems have the ability to access the physical layout whenever they need it. The main advantage in a shared media topology is that the systems have unrestricted access to the physical media. Of course, the main disadvantage to this topology is collisions. If two systems send information out on the wire at the same time, the packets collide and kill both packets. Ethernet is an example of a shared media topology.

To help avoid the collision problem, Ethernet uses a protocol called Carrier Sense Multiple Access/Collision Detection (CSMA/CD). In this protocol, each system monitors the wire, listening for traffic. If traffic is detected, the system waits until it hears no traffic before it sends packets out. If a situation occurs where two systems send out packets at the same time and a collision occurs, each system waits for a period of time before it retries. This time period is different for each system, so that the collision does not occur again.

For small networks, the shared media topology works fine; however, as you begin to add more systems to the network, there is a greater opportunity for collisions. To help reduce the number of collisions, many networks are broken up into several smaller networks with the use of switches or hubs, and each network is then referred to as its own collision domain.

Shared media networks are typically deployed in a bus, star, or hybrid physical topology.

Token Based

The token-based topology works by using a token to provide access to the physical media. In a token-based network, there is a token that travels around the network. When a system needs to send out packets, it grabs the token off of the wire, attaches it to the packets that are sent, and sends it back out on the wire. As the token travels around the network, each system examines the token. When the packets arrive at the destination systems, those systems copy the information off of the wire and the token continues its journey until it gets back to the sender. When the sender receives the token back, it pulls the token off of the wire and sends out a new empty token to be used by the next machine.

Token-based networks do not have the same collision problems that Ethernet-based networks do because of the need to have possession of the token to communicate. However, one problem that does occur with token-based networks is latency. Because each machine has to wait until it can use the token, there is often a delay in when communications actually occur.

Token-based network are typically configured in physical ring topology because the token needs to be delivered back to the originating machine for it to release. The ring topology best facilitates this requirement.

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Thursday, October 1, 2009


The mesh topology is the last topology we discuss. In this layout, every system is connected to every other system. The main advantage of this topology is high availability. The main disadvantage of this topology is cost, both administrative and physical. Because each system is connected to each other, the amount of cabling and maintenance necessary can be prohibitive, especially in larger networks. The formula for determining the amount of cable needed in a mesh network is:

(N x (N - 1))/2, where N is the number of systems to be interconnected

In our example in Figure 5, we have six systems that require 15 cables to create a mesh network. This topology is mainly used in Wide Area Network environments or in environments where high availability outweighs the costs associated with this amount of interconnection.

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Sunday, September 27, 2009


The tree topology is a generalization of the bus topology. The transmission medium is a branching cable with no closed loops. The tree layout begins at a point known as the headend, where one or more cables start, and each of these may have branches. The branches in turn may have additional branches to allow quite complex layouts.

Again, a transmission from any station propagates throughout the medium and can be received by all other stations. Two problems present themselves in this arrangement. First, because a transmission from any one station can be received by all other stations, there needs to be some way of indicating for whom the transmission is intended. Second, a mechanism is needed to regulate transmission.

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Saturday, September 26, 2009


For the bus, all stations attach, through appropriate hardware interfacing known as a tap, directly to a linear transmission medium, or bus. Full-duplex operation between the station and the tap allows data to be transmitted onto the bus and received from the bus.

A transmission from any station propagates the length of the medium in both directions and can be received by all other stations. At each end of the bus is a terminator, which absorbs any signal, removing it from the bus.

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Friday, September 25, 2009

ring topology

In the ring topology, the network consists of a set of repeaters joined by point-topoint links in a closed loop. The repeater is a comparatively simple device, capable of receiving data on one link and transmitting them, bit by bit, on the other link as fast as they are received, with no buffering at the repeater. The links are unidirectional; that is, data are transmitted in one direction only and all are oriented in the same way. Thus, data circulate around the ring in one direction (clockwise or counterclockwise).

Each station attaches to the network at a repeater and can transmit data onto the network through that repeater. As with the bus and tree, data are transmitted in frames. As a frame circulates past all the other stations, the destination station recognizes its address and copies the frame into a local buffer as it goes by. The frame continues to circulate until it returns to the source station, where it is removed. Because multiple stations share the ring, medium access control is needed to determine at what time each station may insert frames.

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Thursday, September 24, 2009


In the star LAN topology, each station is directly connected to a common central node. Typically, each station attaches to a central node, referred to as the star coupler, via two point-to-point links, one for transmission and one for reception. In general, there are two alternatives for the operation of the central node. One approach is for the central node to operate in a broadcast fashion. A transmission of a frame from one station to the node is retransmitted on all of the outgoing links.

In this case, although the arrangement is physically a star, it is logically a bus; a transmission from any station is received by all other stations, and only one station at a time may successfully transmit. Another approach is for the central node to act as a frame switching device. An incoming frame is buffered in the node and then retransmitted on an outgoing link to the destination station.

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Tuesday, September 22, 2009

LAN topology

LAN topology

There are four basic types of LAN topology.


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Wednesday, September 16, 2009

What is the OSI model ?

The OSI model:

* Allows various “open” systems to communicate.
* The Open Systems Interconnection model was created by the International
Standards Organization in the late 1970's.
* Serve as a blueprint for all network communication technologies.
* Dividing up all the processes of networking activity into seven layers.
* Each layer has its own distinct functions and services.

The OSI model consists of seven layers which are:

1. The Physical Layer: transmits raw data bits over a communication channel (mostly
mechanical and electrical issues)
2. The Data Link Layer: guarantees to the network layer that there are no
transmission errors by breaking the input data stream up into frames and sending
back acknowledgement frames
3. The Network Layer: controls the operation of the involved subnet; main issues
are routing (determine a way from source to destination) and dealing with
problems of heterogeneous networks, e. g. different size requirements of
transmitted data blocks
4. The Transport Layer: splits up data from the session layer if necessary
(segmentation) and ensures that the pieces arrive correctly
5. The Session Layer: allows users on different computer systems to establish a
session between them, i. e. they are able to transfer files or log into a remote
system; the conditions of communication are laid down, for example full-duplex
or half-duplex
6. The Presentation Layer: unlike the layers before it is concerned with the syntax
and semantics of the transmitted information; it is concerned with all aspects
of information representation such as data encoding, data compression and
7. The Application Layer: contains a variety of commonly needed protocols like
handling with different terminal types and file systems; a label to identify the
communication process, its origin and destination application is added to the
transmitted information

Layers 4 to 7 are true end-to-end layers, i. e. the layer on the source system carries on a communication process with the same layer on the destination system. In the lower layers the protocols are between a system and its immediate neighbour, for example the source system and a system "on the way" to the destination.

Some of the functions of the physical and the data link layer are combined in the Medium Access Control (MAC) sublayer, which in particular is important to Local Area Networks (LANs). It determines how devices attached to the network gain access to the transmission medium.
Note that the OSI model does not lay down the specific protocols used to communicate between two computers on a specific layer. Although ISO recommends which protocols to use with the OSI model, the model itself is in proper speaking no standard of computer networking. Which protocol in a single layer is actually used, depends on several factors like the physical network, the needed reliability, etc.

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The Importance of Computer Networks

Describes why and how computer networks support successful work

Information and communication are two of the most important strategic issues for the success of every enterprise. While today nearly every organization uses a substantial number of computers and communication tools ( telephones, fax, personal handheld devices), they are often still isolated. While managers today are able to use the newest applications, many departments still do not communicate and much needed information cannot be readily accessed.

To overcome these obstacles in an effective usage of information technology, computer networks are necessary. They are a new kind (one might call it paradigm) of organization of computer systems produced by the need to merge computers and communications. At the same time they are the means to converge the two areas; the unnecessary distinction between tools to process and store information and tools to collect and transport information can disappear. Computer networks can manage to put down the barriers between information held on several (not only computer) systems. Only with the help of computer networks can a borderless communication and information environment be built.

Computer networks allow the user to access remote programs and remote databases either of the same organization or from other enterprises or public sources. Computer networks provide communication possibilities faster than other facilities. Because of these optimal information and communication possibilities, computer networks may increase the organizational learning rate, which many authors declare as the only fundamental advantage in competition.

Besides this major reason why any organization should not fail to have a computer network, there are other reasons as well:

* cost reduction by sharing hard- and software resources
* high reliability by having multiple sources of supply
* cost reduction by downsizing to microcomputer-based networks instead of using
* greater flexibility because of possibility to connect devices from various

Because of the importance of this technology, decisions of purchase, structure, and operation of computer networks cannot be left to technical staff. Management as well has a critical need for understanding the technology of computer networks.

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