Cisco Networking Basics

Some Boolean Logical Operations

2006-05-26T18:03:00.000+03:00

Boolean logic is based on digital circuitry that accepts one or two incoming voltages. Based on the input voltages, output voltage is generated. For computers the voltage difference is represented as an ON or OFF state. These two states are associated with a binary 1 or 0.

AND Operation

The AND operation compares two input values. If both values are 1, the logic gate generates a 1 as the output. Otherwise it outputs a 0. There are four combinations of input values. Three of these combinations generate a 0, and one combination generates a 1. The AND operation represents an electric circuit with two switches arranged in series . If a lamp is added to the circuit, it will only light when BOTH switches are ON.

OR Operation

The OR operation also takes two input values. If at least one of the input values is 1, the output value is 1. Again there are four combinations of input values. Three combinations generate a 1 and the fourth generates a 0. The OR operation represents an electric circuit with two switches arranged in parallel. If a lamp is added to the circuit , it will light if one switch is ON.

NOT Operation

The NOT operation takes the value that is presented and inverts it.A 1 becomes a 0 and a 0 becomes a 1.

The two networking operations that use Boolean logic are subnetwork and wildcard masking. These will be discussed thoroughly in later posts. The next post will discuss briefly the Subnet Mask.

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Addressing your device on networks

2006-05-24T01:57:00.000+03:00

Most devices on a network should have an address to identify them and hence make them reachable. If you are sending data to a certain computer, how will this datagram reach its destination? Simply, this PC will have an address. Actually there are two kinds of addresses assigned for network devices:

Physical Address (MAC Address)

This address is a permenant one which is burnt-in the ROM of the network device ( Network Interface Card{NIC} in case of computers). It cannot be changed unless you replace the device itself . This address is composed of 48 bits which are expressed in the form of 12 Hexadecimal digits. For example:


                   00-30-a5-35-81-4c

Once this address is burnt to a device , it cannot be used or assigned for another device.

The left 6 hexadecimal digits are called the Organizational Unique Identifier as they identify the organization or the company that manufactured this address.
The right 6 hexadecimal digits are called the Vendor Assigned as they identify a serial number in the vendor's line of production .This address has many names including physical, burnt-in, hardware and MAC address(Media Access Control).

Logical Address ( IP Address)

This address is a temporary address which can be changed if wanted . It is assigned to a device using a protocol named Internet Protocol or IP. The address is formed of 32 bits which are expressed in the form of decimal dotted notation to make it easier to work with. This notation is formed of 4 decimal numbers seperated by dots ( Each number being composed originally of 8 bits). For Example :


       200.100.1.2

This Address is composed actually from two portions : a Network ID portion and a Host ID portion . The network id identifies the network to which the host belong , while the host portion identifies the host itself. More information regarding this extremely important point will be given in the next two posts.

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Hexadecimal Numbering System

2006-04-07T18:03:00.001+02:00

The hexadecimal or Base 16 number system is commonly used to represent binary numbers in a more readable form. Computers perform computations in binary. However, the binary output of a computer is expressed in decimal or hexadecimal to make it easier to read.

Like the binary and decimal systems, the hexadecimal system is based on the use of symbols, powers, and positions. The symbols that hexadecimal uses are the digits 0 through 9 and the letters A through F. Where decimal numbers can be represented by any number of bits (8 being the most famous), each Hexadecimal symbol is represented always by 4 bits. Here is the whole table for this numbering system:

 Hexadecimal Symbol                   Corresponding Binary

     0                                    0000
     1                                    0001
     2                                    0010
     3                                    0011
     4                                    0100
     5                                    0101
     6                                    0110
     7                                    0111
     8                                    1000
     9                                    1001
     A                                    1010
     B                                    1011
     C                                    1100
     D                                    1101
     E                                    1110
     F                                    1111

Note: There is an easy way to write the table with no need for summarizing each symbol. Look at the binary columns, The right most column is composed of alternating 0 and 1, the next column( to its left) is composed of alternating TWO 0's and TWO 1's. The next column is composed of alternating FOUR 0's and FOUR 1's and the final column is composed of EIGHT 0's followed by EIGHT 1's.

A hexadecimal number is often indicated with a 0x. For example, the hexadecimal number 2102 would be written as 0x2102. The table above helps you to simply convert from hexadecimal to binary and vice versa. For example:

0x2102 is equivalent to : 0010 0001 0000 0010

11110011 is eqivalent to : 3F hexadecimal.

Note: if you got a binary with number of digits not a multiple of 4, simply add 0's to the left .For example: 011010 , simple add two 0's to the left to become: 00011010 = 1A hexadecimal.

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Coverting decimal to 8-bits binary and vice versa

2006-04-06T23:45:00.000+02:00

There are many methods to convert decimal number into the corresponding binary. Only one famous method will be discussed here:

Converting decimal to binary:

The rules are as follows:

*You should start from the left most binary digit
*If the decimal number is equal or greater than the place value (corresponding 2^number), then place the bit 1 and forward the difference to the next digit on the right
*If the decimal number is less than the place value , then place the bit 0 and forward the number as it is to the next digit on the right

Example:

Suppose you got the decimal number 170:

- 170 is greater than 128(2⁷) , so the left most bit is 1 , get the difference 170-128=42
-forward 42 to the next digit
-42 is less than 64( 2⁶), so place 0 and forward the number as it is (42) to the next step
-42 is greater than 32(2⁵), so place 1 and forward the difference which is 42-32=10
-10 is less than 16(2⁴), so place 0 and forward the 10 as it is
-10 is greater than 8(2³) so place 1 and forward the difference which is 10-8=2
-2 is less than 4( 2²) so place 0 and forward the 2 as it is
-2 is equal 2( 2¹) so place a 1 and forward the difference which is 2-2=0
-0 is less than 1(2⁰) so place a zero and you're done

so the decimal number 170 is equal to the binary number 10101010

NOTE THE FOLLOWING:

*The right most place value is 2⁰

*Whenever you get a zero difference, all the next digits will be o's as zero will be less than 2^whichever number

*Only numbers from 0-255 can be represented by 8 digits binary .Numbers greater than 255 will be represented by more bits. For example: 256 is represented by 9 digits: 100000000. In general 2^n -1 gives you the largest number that can be represented by n digits. So in 8 digits binary, max. number is 2^8 -1= 255. In 9 digits binary, max.number is 2^9-1=511. So the range for 9 digits is from 256-511. Starting from 512 ,10 digits will be required.And so on............

Coverting binary to decimal:

An example about this was already shown in the previous post.

00010110= (1 x 2⁴ = 16) + (0 x 2³ = 0) + (1 x 2² = 4) + (1 x 2¹ = 2) + (0 x 2⁰ = 0) = 22 (16 + 0 + 4 + 2 + 0)

This example shows that the binary number 00010110 is equal to the decimal number 22.

In general , all the 0 bits will add up to zero , so just ignore them and add the 1's. Here is another example:

10101010= (1*2⁷=128) + (1*2⁵=32) + (1*2³ = 8) + (1* 2¹= 2) = 170

( we've already seen that 170 is = 10101010).

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Computer Binary System

2006-03-31T23:48:00.000+02:00

Numbering systems consist of symbols and rules for their use. The most commonly used number system in everday life is decimal, or Base 10.
Base 10 uses the ten symbols 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9. These symbols, can be combined to represent all possible numeric values.

The decimal number system is based on powers of 10. Each column position of a value, from right to left, is multiplied by the base number 10 raised to a power, which is the exponent. The power that 10 is raised to depends on its position to the left of the decimal point. When a decimal number is read from right to left, the first or rightmost position represents 10⁰, which equals 1. The second position represents 10¹, which equals 10. The third position represents 10², which equals 100. The seventh position to the left represents 10⁶, which equals 1,000,000 and so on..

Here is an example:

2134 = (2x10³) + (1x10²) + (3x10¹) + (4x10⁰)

Computers work with and store data using electronic switches that are either ON or OFF. Computers can only understand and use data that is in this two-state or binary format. The 1's and 0's are used to represent the two possible states of an electronic component in a computer. 1 is represented by an ON state, and 0 is represented by an OFF state. They are referred to as binary digits or bits.So computer data are represented by sequences of 1's and 0's ,example: 10001110000101010110101....
Because computers are designed to work with ON/OFF switches, binary digits and binary numbers are natural to them. Humans use the decimal number system, which is relatively simple when compared to the long series of 1's and 0's used by computers.

Bytes

Computers are designed to use groupings of eight bits. This grouping of eight bits is referred to as a byte. In a computer, one byte represents a single addressable storage location. These storage locations represent a value or single character of data, such as an ASCII code.For example: the letter A is represented by 01000010

The binary system uses only two symbols, which are 0 and 1. The position of each digit from right to left in a binary number represents the base number 2 raised to a power or exponent. These place values are, from right to left, 2⁰, 2¹, 2², 2³, 2⁴, 2⁵, 2⁶, and 2⁷, or 1, 2, 4, 8, 16, 32, 64, and 128 respectively.

Here is an example:

10110= (1 x 2⁴ = 16) + (0 x 2³ = 0) + (1 x 2² = 4) + (1 x 2¹ = 2) + (0 x 2⁰ = 0) = 22 (16 + 0 + 4 + 2 + 0)

This example shows that the binary number 10110 is equal to the decimal number 22.

The next article will be dealing with converting decimal numbers to binary and vice versa.

Network Topology

2006-03-24T21:43:00.000+02:00

Network topology defines the structure of the network. One part of the topology definition is the physical topology, which is the actual layout of the wire or media. The other part is the logical topology, which defines how the hosts access the media to send data.

Examples of physical topology:

A bus topology uses a single backbone cable that is terminated at both ends. All the hosts connect directly to this backbone.If this backbone cable is interrupted,all the network will be down.
A ring topology connects one host to the next and the last host to the first. This creates a physical ring of cable.The cables are usually copper cables. Any interruption at any point will lead to failure of the whole netowrk.
A dual ring topology is similar to ring topology but with two rings of cables connected by fibreoptics. This decreases the chance of interruption of service as compared to ring topology.
A star topology connects all cables to a central device(hub,switch,...).If one host fails , the network will still function.
An extended star topology links individual stars together by connecting the hubs or switches.This decrease markedly the chance of interruption of service.
A hierarchical topology is similar to an extended star. However, instead of linking the hubs or switches together, the system is linked to a computer that controls the traffic on the topology.
A mesh topology is implemented to provide as much protection as possible from interruption of service. For example, a nuclear power plant might use a mesh topology in the networked control systems. As seen in the graphic, each host has its own connections to all other hosts. Although the Internet has multiple paths to any one location, it does not adopt the full mesh topology.

Logical Topology:

The logical topology of a network determines how the hosts communicate across the medium. The two most common types of logical topologies are broadcast and token passing.

Broadcast (Non-deterministic,First-come First served) : The use of a broadcast topology indicates that each host sends its data to all other hosts on the network medium. There is no order that the stations must follow to use the network. It is first come, first serve. Signal collison may occur here if by chance two hosts send data at same time.

Token Passing (Deterministic): In this type of topology, an electronic token is passed sequentially to each host. When a host receives the token, that host can send data on the network. If the host has no data to send, it passes the token to the next host and the process repeats itself. This topology works in a collisionless environment as only the host with the token can send data.

Examples of some networks:

Ethernet (most commonly used nowadays) : works with Bus, star or extended star physiscal toplogy and Broadcast logical topology.

Token Ring :as its name implies Ring physical toplogy and Token logical toplogy.

FDDI: Dual ring and Token

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Some Types of Networks

2006-03-23T00:35:00.000+02:00

There are several types of networks which can be categorized according to coverage area,distance, certain features,devices and others.. The following are must-known types:

1)LAN(Local Area Network): This network is characterized by:

- covers a limited geographic area (e.g: a network inside a school,a company,a store,..etc..)

-Always ON

- High Speed Connectivity, eg: fast ethernet works at 100Mbps
-Uses LAN devices as repeators,hubs,bridges and switches.
-Uses LAN technologies as ethernet,Token Ring and FDDI.

2)MAN(Metropolitan Area Network): This network represents a group of LAN's connected together . e.g: a network connecting different subnetwroks over the whole country.

3)WAN(Wide Area Network): This include the Internet.It's characterized by the following:

-covers a large geographic area

-could be Always On as in DSL or On and OFF as in Dial-up modem and ISDN
-Limited speed conncetivity relative to LAN's , eg: DSL max. is 2Mbps.( An exception is Cable Modem which operates at very high speeds.)

-Uses WAN devices as Routers, Modems and Communication Server (Routers are actually both LAN and WAN devices)

Network Requirements

2006-03-21T02:17:00.000+02:00

There are 3 main groups of rquirements for any network:

1)Physical requirements(hardware): including the PC, NIC(network interface card) installed into your PC, media( copper cables, optic fibres, wireless) and netowrk devices like switches, routers, access points,modems....

2) Logical requirements: This is the set of protocols (rules,programs) that are necessary to establish connection between the users.There are lots of protocols, each with certian features and functions.Together these constitute the TCP/IP suite of protocols.
N.B: to test that all protocols are installed correctly, go to your command prompt and type the following: C:/> ping 127.0.0.1
There should be 0% loss of packets.

3) Applications: These are the browsers which are responsible for interpreting and presenting the data. Examples of browsers include: Interent Explorer, Mozilla Firefox and Netscape Navigator. Browsers are able to interpret text and images, however to present media(sound, movies,animations), they need certain plug-ins to be installed.

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