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Friday, September 9, 2011

The OSI Reference Model Explained

OSI stands for the Open Systems Interconnection
The OSI has seven different layers, divided into two groups. The top three layers define how the applications within the end stations will communicate with each other and with users. The bottom four layers define how data is transmitted from one host to the other.

·          _ Application layer (layer 7)
·          _ Presentation layer (layer 6)
·          _ Session layer (layer 5)
·          _ Transport layer (layer 4)
·          _ Network layer (layer 3)
·          _ Data Link layer (layer 2)
·          _ Physical layer (layer 1)
Each layer performs a specialized function to ensure that two hosts or devices effectively communicate, below are some of the functions performed by each layer of the OSI model:
The Application Layer (layer 7)
The application layer provides the user interface. I.e. it marks the spot where users actually communicate to the computer.  Examples of protocols that operate at the application layer include; HTTP (for examples internet browsers work at the application layer), SMTP (for example email clients), FTP (for example FTP clients) and TFTP.

The Presentation Layer (layer 6)
The Presentation layer gets its name from its purpose: It presents data to the Application layer and is responsible for data translation and code formatting. This layer is essentially a translator and provides coding and conversion functions.
Tasks like data compression, decompression, encryption, and decryption are associated with this layer.

The Session Layer (layer 5)
The Session layer is responsible for setting up, managing, and then tearing down sessions between Presentation layer entities. This layer also provides dialog control between communicating hosts. It coordinates communication between systems and serves to organize their communication by offering three different modes: simplex, half duplex, and full duplex.

The Transport Layer (layer 4)
The Transport layer provides end-to-end data transport services and can establish a logical connection between the sending host and destination host on an internetwork.
Examples of protocols operating at this layer include; TCP and UDP, Therefore the Transport layer can be connectionless (UDP) or connection-oriented (TCP).
TCP is considered a reliable mechanism of transport whereas UDP is considered unreliable. The term reliable networking means that acknowledgments, sequencing, and flow control are used. The Transport layer is responsible for providing mechanisms for multiplexing upper-layer applications, establishing sessions, and tearing down virtual circuits.

The Network Layer (layer 3)

The Network layer (also called layer 3) manages IP addressing, tracks the location of hosts on the network, and determines the best way to move data, which means that the Network layer must transport traffic between hosts that aren’t locally attached. Examples of Network layer devices are; Routers and Layer 3 Switches.

The Data Link Layer (layer 2)

The Data Link layer provides the physical transmission of the data, handles error notification, network topology, and flow control. This means that the Data Link layer will ensure that messages are delivered to the proper device on a LAN using hardware or MAC addresses and will translate messages from the Network layer into bits for the Physical layer to transmit. Switches and Bridges are examples of devices that operate at the data link layer.

The Physical Layer (layer 1)

The Physical layer does two things: It sends bits and receives bits. The Physical layer specifies the electrical, mechanical, procedural, and functional requirements for activating, maintaining, and deactivating a physical link between end systems. Examples of devices that operate at the physical layer include; Hubs, NIC (Network Interface card), cables.

Monday, September 5, 2011

What is the difference between classful and classless IP routing protocols?

Classful IP routing protocols

These are routing protocols that do not send subnet mask information when a route update is sent out. Examples are; RIP version 1 and IGRP (Interior Gateway Routing Protocol). Classful routing protocols don’t have a field for subnet information, so the subnet information gets dropped. What this means is that if a router running RIP version 1 or IGRP has a subnet mask of a certain value, it assumes that all interfaces within the classful address space have the same subnet mask.
If you mix subnet mask lengths in a network running RIP or IGRP, that network just won’t work!

Classful network designs are inefficient and wasteful of IP addresses because you have to use the same subnet mask everywhere including point to Point interfaces which would only require two IP addresses.

Another disadvantage of classful routing protocols is that they don’t support summarization of routes and hence the routing tables for such networks tend to grow very big which increase the time a router will take to forward a packet and eventually slows down the network.
Very big routing table are also a head ache to manage and troubleshoot.


Classless IP routing protocols

Classless routing protocols, however, do support the advertisement of subnet information. I.e. these routing protocols send subnet mask information in the routing updates.
Examples of classless routing protocols are; RIP version 2, EIGRP and OSPF.
Therefore, you can use Variable Length Subnet Masks (VLSM) with these routing protocols.
VLSM means we can have different subnet masks for different router interfaces.

This technique helps to save IP address space because you can use subnet masks of different lengths on the different interfaces.

Another advantage of classless routing protocols is that they support summarization of IP routes; this keeps the routing tables in such networks small and easy to manage.
Routers will also spend less time doing route lookups in such small routing tables and hence enhance the forwarding speeds.

Tips on network optimization

There are so many things that can be done by a network administrator to improve network (LAN) performance. I will try and list some from my own experience:

1.   Replace all hubs with switches; A Hub is known to be a single collision domain and floods every frame it receives out all interfaces. It’s up to the computer to select which packet has its MAC address in the header and discard the rest. In turn, switches separate collision domains and use MAC tables to intelligently forward frames to the destination MAC address.

2.   Watch out for loops! Loops in switched networks cause broadcast storms on the network, multiple frame copies which eventually slows down the network and increases the switch CPU utilization. Some techniques used to prevent loops on the network include; CISCO's STP, Extreme Networks's EAPS.

3.   Loose connections; lose connections cause switch ports to keep flapping, this increases network messages being exchange across end points and hence cannibalizing on the bandwidth intended to carry data. Loose connections are caused by bad cable termination and bad patch cords.

4.   Use interfaces with enough capacity at the uplinks; uplinks are the interfaces interconnecting switches to other switches or switches to routers. Make sure that there are no bottle necks at the uplinks. For example gigabit interfaces for large networks and 100mbps interfaces for small networks.

5.   Watch out for computers with viruses on the network. A computer virus will always try to spread over the network and will always be sending unnecessary messages over the network which will clog up your bandwidth. Maintain a virus free network by using a good anti-virus with latest updates.

What is dynamic routing?

Dynamic routing is when protocols are used to find networks and update routing tables on routers as opposed to static routing or default routing. In static routing, IP networks are manually entered into the routing table by use of configuration commands whereas in dynamic routing, the router automatically learns of other neighboring IP networks by use of a routing protocol.
The routing protocols used in dynamic routing include:
1.        RIPv1 - Routing Information Protocol version 1
2.        RIPv2 - Routing Information Protocol version 2
3.        IGRP - Interior Gateway Routing Protocol
4.        EIGRP- Enhanced Interior Gateway Routing Protocol
5.        OSPF – Open Shortest Path First
6.        BGP – Border Gateway Protocol),
7.        MPLS – MultiProtocol Label Switching
8.        ISIS – Intermediate System-to-Intermediate System Protocol

Types of Networking Devices (Hubs, Switches and Routers)

1.   Hubs – These are used to connect devices in the same location into one collision domain. Hubs are not intelligent devices and cause unnecessary congestion on the network.
When a hub receives a frame, it replicates that frame and floods it out on all interfaces looking for the destination. It’s up to the hosts on the network to check the frame header and see who the packet is intended for!
This creates a lot of collisions and the end result is a slow network.
Hubs operate at Layer 1 of the OSI model.

2.   Bridges/Switches – Bridges were used before switches and in fact you can’t find a bridge on the market today. However switches and bridges do the same thing, i.e. breakup collision domains.
Switches create separate collision domains but a single broadcast domain.
Switches are intelligent devices and forward frames based on the destination MAC addresses.
Switches build MAC tables to help them do this intelligent forwarding of frames to destination hosts.
Switches operate at Layer 2 of the OSI model.
Switches are the best option to use when connection devices in a LAN (Local Area Network).

3.   Routers – These operate at Layer 3 of the OSI model and are used for packet switching.
Routers are used to segment broadcast domains.
Routers forward packets based on destination IP address as opposed to switches which use destination MAC addresses.
Routers are used to interconnect separate networks; e.g. Connect a LAN to the internet, Connect two LANs with different subnet ranges.

How to connect two computers using a crossover cable?

To connect two computers without using a Hub or Switch you only need to do two things:
1. Use a crossover cable for this connection; refer to the diagram below to see how the two ends of a crossover cable are terminated.


2. Assign IP addresses to the computers; make sure the two computers are assigned IPs in the same network or subnet.

For example if you choose to use 192.168.1.0/24 as your network address, then:
PC1 à IP Address: 192.168.1.1 Subnet Mask: 255.255.255.0
PC2 à IP Address: 192.168.1.2 Subnet Mask: 255.255.255.0

You can test the connectivity using the usual ping command:
Open the command terminal on your PC.

From PC1: ping 192.168.1.2
From PC2: ping 192.168.1.1

Your connection is successful in you get ping replies with 0% packet loss.
Have fun!!!

Which Subnet Mask gives you 64 Sub-networks from a class B network address?

In other words you want to take one network address and create 64 sub-networks from it!
To create sub-networks, you take bits from the host portion of the IP address and reserve them to define the subnet address.

Default subnet mask for all class B addresses is 255.255.0.0 or 11111111.11111111.00000000.00000000 or /16 where the “1”s define the network portion and “0”s define the host portion.

Number of subnets = 2^n
Where n is the number of “1”s
ð  64= 2^n
ð  2^6 = 2^n
ð  6 = n                                                                                                                                                                                                               
Which means we shall take 6 “0”s from the host portion and convert them to “1”s; The new subnet mask will be 11111111.11111111.11111100.00000000 or 255.255.252.0 or /22
Answer = 255.255.252.0 or /22
Leaving only 10 “0”s to define the hosts;
Number of Hosts = (2^m) - 2
Where m = number of “0”s
ð  Number of Hosts = (2^10) -2
ð  Number of Hosts = 1024 -2 =1022
You need to subtract 2 for the subnet address and the broadcast address, which are not valid hosts.

Monday, August 29, 2011

Create, Resize and Merge Partitions on Your Hard Drive - Use Partition Magic

Partition Magic

This software allows you to; create, resize and merge partitions on your hard drive without destroying data. It also helps organize and protect your data.

Partition Magic Features:

· Organize and protect the information on a hard drive
· Easy, step-by-step wizards help with performing partitioning tasks
· Preview the effects of partitioning a hard drive before implementing anything
· Partition hard drives as large as 80 GB
· Access and partition external USB drives
· Undelete a deleted partition from rescue disks
· Remotely copy, move, delete, and create partitions across a TCP/IP connection
· Create and run scripts to automate common partitioning tasks
· Shred partitions to ensure the security of corporate data on retiring hard disks
· Convert from one file system or partition type to another without losing data
· Supports FAT, FAT32, NTFS, and Linux ext2/Swap file systems
· Support for Windows XP

Sunday, August 28, 2011

Troubleshooting Network Connectivity Issues - The PING Command

PING

PING stands for Packet INternet Grouper.
It’s a tool available in computers and network devices to help in troubleshooting connectivity issues.
Two computers are said to be successfully connected at the network level if they can successfully PING each other.

The ping command is usually supplied with the destination IP or hostname of the PC you wish to ping.

e.g.
#ping 192.168.2.1

C:\Program Files\ICW>ping www.yahoo.com

Pinging any-fp3-real.wa1.b.yahoo.com [69.147.125.65] with 32 bytes of data:
Reply from 69.147.125.65: bytes=32 time=459ms TTL=52
Reply from 69.147.125.65: bytes=32 time=475ms TTL=52
Reply from 69.147.125.65: bytes=32 time=451ms TTL=52
Reply from 69.147.125.65: bytes=32 time=467ms TTL=52

Ping statistics for 69.147.125.65:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
    Minimum = 451ms, Maximum = 475ms, Average = 463ms

C:\Program Files\ICW>
A ping is successful if it returns replies from the remote/destination PC/host.

Examples of broadband technologies

Broadband Technologies

Broadband Technologies are of two kinds:

1.        Fixed line technologies
2.        Wireless technologies

Below are examples in each category;

Examples of broadband fixed line technologies include:

(i)                    DSL – Digital Subscriber Line
(ii)                  BPL – Broadband Power Line

Examples of broadband wireless technologies include:

(i)                    WiFi – Wireless Fidelity
(ii)                  WiMax – Worldwide Interoperability for Microwave Access
(iii)                 3G – 3rd Generation Mobile Network
(iv)                 CDMA – Code Division Multiple Access
(v)                   Microwave Links
(vi)                 Satellite

CCNA - Understanding a class B network address

Class B Network Address

The designers of the Internet decided to create classes of networks based on network size.
A “class B” network address has its first 2 octets (2bytes or 16 bits) assigned to identify the network and the last 2 octets (16 bits) assigned to identify a host on that network.

If     N = Network bit
   H = Host bit

Class A: NNNNNNNN.HHHHHHHH.HHHHHHHH.HHHHHHHH
Class B: NNNNNNNN.NNNNNNNN.HHHHHHHH.HHHHHHHH
Class C: NNNNNNNN.NNNNNNNN.NNNNNNNN.HHHHHHHH

A bit can take on a two values (0 or 1), meaning;

Class A can have up to 2^8 = 256 networks and 2^24 = 16,777,216 hosts.
Class B can have up to 2^16 = 65,536 networks and 2^16 = 65,536 hosts.
Class C can have up to 2^24 = 16,777,216 networks and 2^8 = 256 hosts.

For the small number of networks possessing a very large number of nodes, they created the rank “Class A” network. At the other extreme is the “Class C” network, which is reserved for the numerous networks with a small number of nodes. The class distinction for networks between very large and very small is predictably called the “Class B” network.

In a “Class B” network, the first bit of the first byte must always be turned ON (set to 1) but the second bit must always be turned OFF (set to 0). If you turn the other 6 bits all OFF and then all ON, you will find the range for a Class B network:

10000000 = 128
10111111 = 191

As you can see, a Class B network is defined when the first byte is configured from 128 to 191.

Friday, August 12, 2011

How to connect your PC/Laptop to your TV/LCD

Today new TVs/LCDs come with the PC VGA input port and it’s very easy to connect your laptop to your giant screen and enjoy the movies you saved on your hard disk. All you need for this simple setup is a VGA cable; connect one end to the PC VGA port and the other end to your TV and select PC input from your TV input menu.




However, some people still own the old model TVs which don’t have the VGA port! The good news it is still possible to connect your PC or Laptop to your Television set and also enjoy movies, pictures, videos from your PC/Laptop.
For this kind of setup, you need a “PC VGA to TV S-Video”.

The converter comes with the VGA cable and S-Video cable.





See how you will hook it up in the diagram below:

Have fun!

Thursday, August 11, 2011

Upgrade your "swag" with the Voosoo V7 Android Tablet - Has it all, yet Cheaper

Are you still on the hunt for an android PC tablet that has it all yet gentle to your pockets? I found the "Voosoo V7 Android Telephony Pad".
This 7 iPAD is a 3G telephone and a computer (“Kill two birds with one stone”).

This tablet is thin, sleek, high-end and portable. It’s the real deal.
Get a feel of the android 2.2 operating system; smoother control, easier web browsing and quick navigation.

Experience superfast 3G + Wi-Fi internet access on this 7-inch capacitive touch screen.

Other niceties include; support flash 10.1 for online video playback, Mobile Office to easily read and edit word, pdf, ppt, Excel and other documents, Camera (front and back),GPS, Email, Picture browsing, Music player and Video player.

See detailed product specifications here: http://www.voosoo.com.cn/product/en/productshow.asp?id=716

Thursday, June 30, 2011

Why you should know the Google Page Rank of your Website or Blog?

Check the Google Page Rank of your website here:


Check Page Rank of your Web site pages instantly:




This page rank checking tool is powered by Page Rank Checker service
Google PageRank also abbreviated as “PR” is an algorithm used by the Google search engine to measure the relevance of a page, website or blog on the World Wide Web (www). In other words PageRank is how Google decides the importance of a web page or website.

Monday, June 20, 2011

Microsoft Word Keyboard Shortcuts

How to select all data?
Press and hold [Ctrl] + [A]
How to a select a portion of data in MS word?
Press and hold [Shift] and then use the [UP/DOWN/RIGHT/LEFT] arrow keys to select the data.
How to copy the selected data?
Press and hold [Ctrl] + [C]
How to cut the selected data?
Press and hold [Ctrl] + [X]
How to paste the selected data?
Press and hold [Ctrl] + [V]
How to underline the selected data?
Press and hold [Ctrl] + [U]
How to bold the selected data?
Press and hold [Ctrl] + [B]
How to apply italics on the selected data?
Press and hold [Ctrl] + [I]
How to undo?
Press and hold [Ctrl] + [Z]
How to select fonts?
Press and hold [Ctrl] + [Shift] + [F], this activates the Font list box in the Formatting toolbar.
Press the
[Down] Arrow or [Up] Arrow keys to select the font you want to use.
Press
[Enter] to apply the font.
How to open a new document without closing the other?
Press and hold [Ctrl] + [N]
How to save?
Press and hold [Ctrl] + [S]
How to close the active document or window?
Press and hold [Alt] + [F4]
Or
Press and hold [Alt] + [spacebar] + [C]
 How Do I Minimize An Active Window?
Press and hold [Alt] + [spacebar] + [N]
How Do I Maximize An Active Window?
Press and hold [Alt] + [spacebar] + [X]
How Do I Restore An Active Window?
Press and hold [Alt] + [spacebar] + [R]
How Can I select a Check Box or Options Button?
Use the [Tab] key to move around a dialog box or options window.
Activate the check box or options button (using the
[Tab] key).
To select the check box or options button; press the
[spacebar].
How Can I move around a dialog box or an options window?
Use the [Tab] key to move around a dialog box or options window.
How to move the cursor without the mouse?
Press and hold [ALT] + [SHIFT] + [Num Lock]
This will activate MouseKeys, enabling you to use [8],[4],[6] and [2] to control the cursor.
To be continued