Hub

 

What is a hub?

A hub is an element of hardware for centralising network traffic coming from multiple hosts, and to propagate the signal. The hub has a certain number of ports (it has enough ports to link machines to one another, usually 4, 8, 16 or 32). Its only goal is to recover binary data coming into a port and send it to all the other ports. As with a repeater, a hub operates on layer 1 of the OSI model, which is why it is sometimes called a multiport repeater.

hub

The hub connects several machines together, sometimes arranged in a star shape, which gives it its name, due to the fact that all communication coming from the machines on the network passes through it.
Types of hubs

There are several categories of hubs:

    * "Active" hubs: They are connected to an electrical power source and are used to refresh the signal being sent to the ports.

    * "Passive" ports: They simply send the signal to all the connected hosts, without amplifying it.


Connecting multiple hubs

It is possible to connect several hubs together in order to centralise a larger number of machines; this is sometimes called a daisy chain. To do this, all that is needed is to connect the hubs using crossover cable, a kind of cable which links the in/out ports on one end to those on the other.

Hubs generally have a special port called an "uplink" for connecting two hubs together using a patch cable. There are also hubs which can cross or uncross their ports automatically depending on whether they are connected to a host or a hub.

setting up a daisy chain


Note:     Up to three hubs can be chained.

If you want to connect multiple machines to your Internet connection, a hub is not enough. You'll either need to have a router or a switch, or to leave the computer connected directly as a gateway (it will stay on constantly for as long as the other computers on the network want to access the Internet.)
 

WiFi

 

Introduction to Wi-Fi (802.11)

The IEEE 802.11 specification (ISO/IEC 8802-11) is an international standard describing the characteristics of a wireless local area network (WLAN). The name Wi-Fi (short for "Wireless Fidelity", sometimes incorrectly shortened to WiFi) corresponds to the name of the certification given by the Wi-Fi Alliance, formerly WECA (Wireless Ethernet Compatibility Alliance), the group which ensures compatibility between hardware devices that use the 802.11 standard. Today, due to misuse of the terms (and for marketing purposes), the name of the standard is often confused with the name of the certification. A Wi-Fi network, in reality, is a network that complies with the 802.11 standard. Hardware devices certified by the Wi-Fi Alliance are allowed to use this logo:

Wi-Fi Certification Logo

With Wi-Fi, it is possible to create high-speed wireless local area networks, provided that the computer to be connected is not too far from the access point. In practice, Wi-Fi can be used to provide high-speed connections (11 Mbps or greater) to laptop computers, desktop computers, personal digital assistants (PDAs) and any other devices located within a radius of several dozen metres indoors (in general 20m-50m away) or within several hundred metres outdoors.

Wi-Fi providers are starting to blanket areas that have a high concentration of users (like train stations, airports, and hotels) with wireless networks. These access areas are called "hot spots".

Introduction to Wi-Fi (802.11)

The 802.11 standard reserves the low levels of the OSI model for a wireless connection that uses electromagnetic waves, i.e.:

    * The physical layer (sometimes shortened to the "PHY" layer), which offers three types of information encoding.
    * The data link layer, comprised of two sub-layers: Logical Link Control (or LLC) and Media Access Control (or MAC).

The physical layer defines the radio wave modulation and signalling characteristics for data transmission, while the data link layer defines the interface between the machine's bus and the physical layer, in particular an access method close to the one used in the Ethernet standard and rules for communication between the stations of the network. The 802.11 standard actually has three physical layers, which define alternative modes of transmission:

Data Link Layer

(MAC)     802.2   802.11

Physical Layer
(PHY)    
DSSS    FHSS    Infrared

Any high-level protocol can be used on a Wi-Fi wireless network the same way it can be used on an Ethernet network.

The various Wi-Fi standards

The IEEE 802.11 standard is actually only the earliest standard, allowing 1-2 Mbps of bandwidth. Amendments have be made to the original standard in order to optimise bandwidth (these include the 802.11a, 802.11b and 802.11g standards, which are called 802.11 physical standards) or to better specify components in order to ensure improved security or compatibility. This table shows the various amendments to the 802.11 standard and their significance:

Name of standard    Name    Description

 
802.11a     Wifi5     The 802.11a standard (called WiFi 5) allows higher bandwidth (54 Mbps maximum throughput, 30 Mbps in practice). The 802.11a standard provides 8 radio channels in the 5 GHz frequency band.
802.11b    WiFi    The 802.11b standard is currently the most widely used one. It offers a maximum thoroughput of 11 Mbps (6 Mbps in practice) and a reach of up to 300 metres in an open environment. It uses the 2.4 GHz frequency range, with 3 radio channels available.
802.11c    Bridging 802.11 and 802.1d    The 802.11c bridging standard is of no interest to the general public. It is only an amended version of the 802.1d standard that lets 802.1d bridge with 802.11-compatible devices (on the data link level).
802.11d    Internationalisation    The 802.11d standard is a supplement to the 802.11 standard which is meant to allow international use of local 802.11 networks. It lets different devices trade information on frequency ranges depending on what is permitted in the country where the device is from.
802.11e    Improving service quality    The 802.11e standard is meant to improve the quality of service at the level of the data link layer. The standard's goal is to define the requirements of different packets in terms of bandwidth and transmission delay so as to allow better transmission of voice and video.
802.11f    Roaming    The 802.11f is a recommendation for access point vendors that allows products to be more compatible. It uses the Inter-Access Point Roaming Protocol, which lets a roaming user transparently switch from one access point to another while moving around, no matter what brands of access points are used on the network infrastructure. This ability is also simply called roaming.
802.11g         The 802.11g standard offers high bandwidth (54 Mbps maximum throughput, 30 Mbps in practice) on the 2.4 GHz frequency range. The 802.11g standard is backwards-compatible with the 802.11b standard, meaning that devices that support the 802.11g standard can also work with 802.11b.
802.11h         The 802.11h standard is intended to bring together the 802.11 standard and the European standard (HiperLAN 2, hence the h in 802.11h) while conforming to European regulations related to frequency use and energy efficiency.
802.11i         The 802.11i standard is meant to improve the security of data transfers (by managing and distributing keys, and implementing encryption and authentication). This standard is based on the AES (Advanced Encryption Standard) and can encrypt transmissions that run on 802.11a, 802.11b and 802.11g technologies.
802.11Ir         The 802.11r stadard has been elaborated so that it may use infra-red signals. This standard has become technologically obsolete.
802.11j         The 802.11j standard is to Japanese regulation what the 802.11h is to European regulation.
It is also useful to note the existence of a standard called "802.11b+". This is a proprietary standard with improvements in data flow. However, this standard also suffers from gaps in interoperability due to not being an IEEE standard.
Range and data flow

The 802.11a, 802.11b and 802.11g standards, called "physical standards" are amendments to the 802.11 standard and offer different modes of operation, which lets them reach different data transfer speeds depending on their range.

Standard               Frequency     Speed         Range

 
WiFi a (802.11a)     5 GHz         54 Mbit/s     10 m

WiFi B (802.11b)     2.4 GHz     11 Mbit/s     100 m

WiFi G (802.11b)     2.4 GHz     54 Mbit/s     100 m

802.11a

The 802.11 standard has a maximum theoretical data flow of 54 Mbps, five times that of 802.11b, but at a range of only about thirty metres. The 802.11a standard relies on a technology called OFDM (Orthogonal Frequency Division Multiplexing). It broadcasts in the 5 GHz frequency range and uses 8 non-overlapping channels.

Because of this, 802.11a devices are incompatible with 802.11b devices. However, there are devices that incorporate both 802.11a and 802.11b chips, called "dual band" devices.

Hypothetical speed

(indoors)      Range

54 Mbits/s     10 m

48 Mbits/s     17 m

36 Mbits/s     25 m

24 Mbits/s     30 m

12 Mbits/s     50 m

6 Mbits/s     70 m

802.11b

The 802.11b standard allows for a maximum data transfer speed of 11 Mbps, at a range of about 100 m indoors and up to 200 metres outdoors (or even beyond that, with directional antennas.)

Hypothetical speed    Range

(indoors)    Range   (outdoors)

11 Mbits/s     50 m     200 m

5.5 Mbits/s     75 m     300 m

2 Mbits/s     100 m     400 m

1 Mbit/s     150 m     500 m

802.11g

The 802.11g standard allows for a maximum data transfer speed of 54 Mbps at ranges comparable to those of the 802.11b standard. What's more, as the 802.11g standard uses the 2.4GHz frequency range with OFDM coding, this standard is compatible with 802.11b devices, with the exception of some older devices.

Hypothetical speed    Range

 
(indoors)      Range (outdoors)

 
54 Mbits/s     27 m     75 m

48 Mbits/s     29 m     100 m

36 Mbits/s     30 m     120 m

24 Mbit/s     42 m     140 m

18 Mbit/s     55 m     180 m

12 Mbit/s     64 m     250 m

9 Mbit/s     75 m     350 m

6 Mbit/s     90 m     400 m
 

Fire wall

What is a Firewall?

A firewall is a system that protects a computer or a computer network against intrusions coming from a third-party network (generally the Internet). A firewall is a system that filters data packets that are exchanged over the network.

 

• an interface for the network being protected (internal network)
• an interface for the external network
  

The firewall system is a software system, often supported by dedicated network hardware, forming an intermediary between the local network(or the local computer) and one or more external networks. A firewall system can be set up on any computer that uses any system as long as:

• The machine is powerful enough to process the traffic
• The system is secure
• No other service other than the packet filtering service is running on the server

In the case that a firewall system is provided in a black box, the term "appliance" applies.

How a Firewall System Works

 

 A firewall system contains a set of predefined rules that allow the system to: 

 

• Authorise the connection (allow)
• Block the connection (deny)
• Reject the connection request without informing the issuer (drop)

All of these rules implement a filtering method that depends on the security policy that was adopted by the organisation. Security policies are usually broken down into two types that allow:

• the authorisation of only those communications that were explicitly authorised:
  
  
  
  
  
  
  
  

  "Everything that is not explicitly authorised is prohibited"

  
  
  
  
  
  
  
  
  
  
  
• the refusal of exchanges that were explicitly prohibited

The first method is without a doubt the safest. However, it imposes a precise and restrictive definition of communication needs.

 

Hard disk

                              Hard drive

The hard drive is the component which is used to permanently store data, as opposed to RAM, which is erased whenever the computer is restarted, which is why the term mass storage device is sometimes used to refer to hard drives.

The hard drive is connected to the motherboard using a hard drive controller which acts as an interface between the processor and the hard drive.

Structure

 A hard drive is made up of not just one, but several rigid metal, glass, or ceramic disks, stacked very close to one another and called platters. 

The disks turn very quickly around an axle (currently several thousand revolutions per minute) in a counter-clockwise direction .

The data is stored in the form of 0s and 1s (called bits).

Hard drives hold millions of these bits, stored very close to one another on a fine magntic layer a few microns thick, which is covered by a protective film. 

They are read and written using read heads located on both sides of the platters. These heads are electromagnets which raise and lower themselves in order to read or write data. The read heads are only a few microns from the surface, separated by a layer of air created by the rotation of the disks, which generates a wind of about 250km/h (150 mph)! What's more, these disks are laterally mobile, so that the heads can sweep across their entire surface. 

 

However, the heads are linked to one another and only one of them can read or write at a given moment. The term cylinder is used to refer to all the data stored vertically on each of the disks.

This entire precision mechanism is contained within a fully airtight case, as the smallest particle can degrade the disk's surface. This is why hard drives are closed shut with seals, and the warning "Warranty void if removed", as only hard drive manufacturers can open them (in particle-free "cleanrooms").

How it works

 

The read/write heads are said to be "inductive", meaning that they can generate a magnetic field. This is especially important in writing: The heads, by creating positive or negative fields, polarise the disk surface in a very tiny area, so that when they are read afterwards, the polarity reversal completes a circuit with the read head, which is then transformed by an analog-digital converter (ADC) into a 0 or 1 which can be understood by the computer.

 

The heads start writing data from the edge of the disk (track 0), then move onward towards the centre. The data is organised in concentric circles called "tracks", which are created by  low level formatting.

The tracks are separated into areas (between two radii) called sectors, containing data (generally at least 512 octets per sector).

  

The term cylinder refers to all data found on the same track of different platters (i.e. above and below one another), as this forms a "cylinder" of data.

Finally, the term clusters (also called allocation units) refers to minimum area that a file can take up on the hard drive. An operating system uses blocks, which are in fact groups of sectors (between 1 and 16 sectors). A small file may occupy multiple sectors (a cluster).

On old hard drives, addressing was done physically, by defining the position of the date from the coordinates Cylinder/Head/Sector (CHS). 

Technical specifications

• Capacity: Amount of data which can be stored on a hard drive.
• Transfer rate: Quantity of data which can be read or written from the disk per unit of time. It is expressed in bits per second.
• Rotational speed: The speed at which the platters turn, expressed in rotations per minute (rpm for short). Hard drive speeds are on the order of 7200 to 15000 rpm. The faster a drive rotates, the higher its transfer rate. On the other hand, a hard drive which rotates quickly tends to be louder and heats up more easily.
• Latency (also called rotational delay): The length of time that passes between the moment when the disk finds the track and the moment it finds the data.
• Average access time: Average amount of time it takes the read head to find the right track and access the data. In other words, it represents the average length of time it takes the disk to provide data after having received the order to do so. It must be as short as possible.
• Radial density: number of tracks per inch (tpi).
• Linear density: number of bits per inch (bpi) on a given track.
• Surface density: ratio between the linear density and radial density (expressed in bits per square inch).
• Cache memory (or buffer memory): Amound of memory located on the hard drive. Cache memory is used to store the drive's most frequently-accessed data, in order to improve overall performance; 
  
• Interface: This refers to the connections used by the hard drive. The main hard drive interfaces are:
  • IDE/ATA
    
  • SERIAL ATA
    
  • SCSI
    
  • However, there are external cases used for connecting hard drives with USB or FIRE WIRE ports.

 

Random Access Memory

random access memory

A type of computer that can be accessed randomly; that is, any byte of memory can be accessed without touching the preceding bytes. 

Types of random access memory

• DRAM memories (Dynamic Random Access Module), which are inexpensive. They are used essentially for the computer's main memory.
• SRAM memories (Static Random Access Module), which are fast and costly. SRAM memories are used in particular for the processors cache memory.

 

Operation of the random access memory

 The random access memory comprises hundreds of thousands of small capacitors that store loads. When loaded, the logical state of the capacitor is equal to 1, otherwise it is 0, meaning that each capacitor represents one memory bit.
 

Given that the capacitors become discharged they must be constantly recharged (the exact term is refresh) at regular intervals, known as the refresh cycle. DRAM memories for example require refresh cycles of around 15 nanoseconds (ns). 

Each capacitor is coupled with a transistor (MOS-type) enabling "recovery" or amendment of the status of the capacitor. These transistors are arranged in the form of a table (matrix) thus we access a memory box (also called memory point) via a line and a column.  

Each memory point is thus characterised by an address which corresponds to a row number and a column number. This access is not instant and the access time period is known as latency time. Consequently, time required for access to data in the memory is equal to cycle time plus latency time.

Thus, for a DRAM memory, access time is 60 nanoseconds (35ns cycle time and 25ns latency time). On a computer, the cycle time corresponds to the opposite of the clock frequency; for example, for a computer with frequency of 200 MHz, cycle time is 5 ns (1/200*10⁶)).

Consequently a computer with high frequency using memories with access time much longer than the processor cycle time must perform wait states to access the memory. For a computer with frequency of 200 MHz using DRAM memories (and access time of 60ns), there are 11 wait states for a transfer cycle. The computer's performance decreases as the number of wait states increases, therefore we recommend the use of faster memories.

 

What is BUS...?

Bus

*)     Bus is nothing but the cables, printed circuits, etc.


*)     A bus, in computing, is a set of physical connections which can be shared by multiple hardware components in order to communicate with one another. 


*)   The purpose of buses is to reduce the number of "pathways" needed for communication between the components, by carrying out all communications over a single data channel. This is why the metaphor of a "data highway" is sometimes used.

Characteristics of a bus

A bus is characterised by the amount of information that can be transmitted at once.

This amount, expressed in bits, corresponds to the number of physical lines over which data is sent simultaneously.

A 32-wire ribbon cable can transmit 32 bits in parallel.

The term "width" is used to refer to the number of bits that a bus can transmit at once. 

The bus speed is also defined by its frequency (expressed in Hertz), the number of data packets sent or received per second.

Each time that data is sent or received is called a cycle. 

This way, it is possible to find the maximum transfer speed of the bus, the amount of data which it can transport per unit of time, by multiplying its width by its frequency.

A bus with a width of 16 bits and a frequency of 133 MHz, therefore, has a transfer speed equal to: 

 

16 * 133.106 = 2128*106 bit/s,
or 2128*106/8 = 266*106 bytes/s
or 266*106 /1000 = 266*103 KB/s
or 259.7*103 /1000 = 266 MB/s

Bus sub assembly

• The address bus (sometimes called the memory bus) transports memory addresses which the processor wants to access in order to read or write data. It is a unidirectional bus.
• The data bus transfers instructions coming from or going to the processor. It is a bidirectional bus.
• The control bus (or command bus) transports orders and synchonisation signals coming from the control unit and travelling to all other hardware components. It is a bidirectional bus, as it also transmits response signals from the hardware.
  

 

what is port...?

*)    An interface on a computer to which you connect a device.

*) Internally there are several port for connecting                                 disk drive,display screen and key board.

*)  Externally personal computer have port for modem, printer and peripheral devices.

*) Almost all personal computers come with a serial RS-232C  port or RS-422  port for connecting a modem or mouse and a parallel port  for connecting a printer. 

*) The parallel port uses a 25-pin connector (type DB-25) and is used to connect printers.

Transmission Control Protocol (TCP)

------------------------------------------------------------------------------------------------------

TCP Header

------------------------------------------------------------------------------------------------------

Bit offset	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29	30	31

 ------------------------------------------------------------------------------------------------------

0

Source       port

Destination     port

 ------------------------------------------------------------------------------------------------------

     32               Sequence number 

 ------------------------------------------------------------------------------------------------------

     64                Acknowledgment number 

 ------------------------------------------------------------------------------------------------------

96   Data offset

Reserved

C W R

E C E

U R G

A C K

P S H

R S T

S Y N

F I N

window Size

 ------------------------------------------------------------------------------------------------------

128

Urgent pointer

 ------------------------------------------------------------------------------------------------------

  160             options(if data offset>5)

 

 ------------------------------------------------------------------------------------------------------

Tcp  is on of the core protocal for all protocal suite.

TCP operates at a higher level, concerned only with the two end systems.

TCP segment structure

• Source port (16 bits) – identifies the sending port
• Destination port (16 bits) – identifies the receiving port
• Sequence number (32 bits) – has a dual role

• If the SYN flag is set, then this is the initial sequence number. The sequence number of the actual first data byte (and the acknowledged number in the corresponding ACK) will then be this sequence number plus 1.
• If the SYN flag is clear, then this is the sequence number of the first data byte

• Acknowledgment number (32 bits) – if the ACK flag is set then the value of this field is the next sequence number that the receiver is expecting. This acknowledges receipt of all prior bytes (if any). The first ACK sent by each end acknowledges the other end's initial sequence number itself, but no data.
• Data offset (4 bits) – specifies the size of the TCP header in 32-bit words. The minimum size header is 5 words and the maximum is 15 words thus giving the minimum size of 20 bytes and maximum of 60 bytes, allowing for up to 40 bytes of options in the header. This field gets its name from the fact that it is also the offset from the start of the TCP segment to the actual data.
• Reserved (4 bits) – for future use and should be set to zero
• Flags (8 bits) (aka Control bits) – contains 8 1-bit flags

CWR (1 bit) – Congestion Window Reduced (CWR) flag is set by the sending host to indicate that it received a TCP segment with the ECE flag set and had responded in congestion control mechanism

• ECE (1 bit) – ECN-Echo indicates

• If the SYN flag is set, that the TCP peer is ECN capable.
• If the SYN flag is clear, that a packet with Congestion Experienced flag in IP header set is received during normal transmission

• (1 bit) – indicates that the Urgent pointer field is significant
• ACK (1 bit) – indicates that the Acknowledgment field is significant. All packets after the initial SYN packet sent by the client should have this flag set.
• PSH (1 bit) – Push function
• RST (1 bit) – Reset the connection
• SYN (1 bit) – Synchronize sequence numbers. Only the first packet sent from each end should have this flag set. Some other flags change meaning based on this flag, and some are only valid for when it is set, and others when it is clear.
• FIN (1 bit) – No more data from sender

Window (16 bits) – the size of the receive window, which specifies the number of bytes (beyond the sequence number in the acknowledgment field) that the receiver is currently willing to receive

 

• Checksum (16 bits) – The 16-bit checksum field is used for error-checking of the header and data
•  
• Urgent pointer (16 bits) – if the URG flag is set, then this 16-bit field is an offset from the sequence number indicating the last urgent data byte
•  
• Options (Variable 0-320 bits, divisible by 32) – The length of this field is determined by the data offset field. Options 0 and 1 are a single byte (8 bits) in length. The remaining options indicate the total length of the option (expressed in bytes) in the second byte. Some options may only be sent when SYN is set; they are indicated below as .
•  

• 0 (8 bits) - End of options list
•  
• 1 (8 bits) - No operation (NOP, Padding) This may be used to align option fields on 32-bit boundaries for better performance.
•  
• 2,4,SS (32 bits) - Maximum segment size 
• 
  
• 3,3,S (24 bits) - Window scale
•  
• 4,2 (16 bits) - Selective Acknowledgement supported. 
•  
• 5,N,BBBB,EEEE,... (variable bits, N is either 10, 18, 26, or 34)- Selective ACKnowlegement (SACK) These first two bytes are followed by a list of 1-4 blocks being selectively acknowledged, specified as 32-bit begin/end pointers.
•  
• 8,10,TTTT,EEEE (80 bits)- Timestamp and echo of previous timestamp 
• 
  
• 14,3,S (24 bits) - TCP Alternate Checksum Request. 
• 
  
• 15,N,... (variable bits) - TCP Alternate Checksum Data. 

 

Protocol operation

 

• LISTEN : In case of a server, waiting for a connection request from any remote client. 
•  
• SYN-SENT : waiting for the remote peer to send back a TCP segment with the SYN and ACK flags set. (usually set by TCP clients)
•  
• SYN-RECEIVED : waiting for the remote peer to send back an acknowledgment after having sent back a connection acknowledgment to the remote peer. (usually set by TCP servers)
•  
• ESTABLISHED : the port is ready to receive/send data from/to the remote peer.
•  
• FIN-WAIT-1
•  
• FIN-WAIT-2
•  
• CLOSE-WAIT
•  
• CLOSING
•  
• LAST-ACK
• TIME-WAIT : represents waiting for enough time to pass to be sure the remote peer received the acknowledgment of its connection termination request. According to RFC 793 a connection can stay in TIME-WAIT for a maximum of four minutes.
•  
• CLOSED

What is SPAM..?

* E- mail  spam  is  also  known as  Junk-Mail . It is a subset of spam that involves nearly identical message to the recipiants by email.

* A common synonym for spam is unsolicited bulk e-mail

Mail Bomb

*  e-mail bomb is a form of net abuse consisting of sending huge volumes of e-mail to an address in an attempt to overflow the mailbox or overwhelm the server where the email address is hosted in a denial of service attack.

There are two types are there,

1)  Mass mailing

2) list linking

MASS  MAILING :

Mass mailing consists of sending numerous duplicate mails to the same email address.

LIST LINKING :

List linking means signing a particular email address up to several email list subscriptions. The victim then has to unsubscribe from these unwanted services manually.

Hacking

 

HACKING :

     Hacking is unauthorized use of computer and network resources. 

HACKER: 

a hacker is a person who breaks into computers, usually by gaining access to administrative controls

Hacker attitudes :

 

 

 BLACK HAT:

 he breaks computer security without authorization or uses technology (usually a computer, phone system or network) for vandalism, credit card fraud, identity theft, piracy, or other types of illegal activity.

 WHITE HAT: 

A white hat hacker breaks security for non-malicious reasons, for instance testing their own security system. 

This type of hacker enjoys learning and working with computer systems, and consequently gains a deeper understanding of the subject.

Such people normally go on to use their hacking skills in legitimate ways.

 GREY HAT:

A grey hatted hacker is a hacker of ambiguous ethics and/or borderline legality, often frankly admitted.

 Script kiddie :

A script kiddie is a non-expert who breaks into computer systems by using pre-packaged automated tools written by others, usually with little understanding. These are the outcasts of the hacker community.

 Hacktivist

A hacktivist is a hacker who utilizes technology to announce a social, ideological, religious, or political message. 

In general, most hacktivism involves website defacement or denial-of-service attacks.

 Techniques used by computer criminals and security experts.

Vulnerability scanner :

A vulnerability scanner is a tool used to quickly check computers on a network for known weaknesses. 

Hackers also commonly use port scanner.

These check to see which ports on a specified computer are "open" or available to access the computer, and sometimes will detect what program or service is listening on that port, and its version number.

Password cracking:

 Password cracking is the process of recovering password from data that has been stored in or transmitted by a computer system.

A common approach is to repeatedly try guesses for the password.

Packet sniffer :

A packet analyzer (also known as a network analyzer, protocol analyzer or sniffer, or for particular types of networks an Ethernet sniffer or wireless sniffer) is computer software or computer hardware that can intercept and log traffic passing over a digital network or part of a network. As data stream flow across the network, the sniffer captures each packet and ,if needed, decodes and analyzes its content according to the appropriate RFC or other specifications.

 Spoofing attack:

 spoofing attack is a situation in which one person or program successfully masquerades as another by falsifying data and thereby gaining an illegitimate advantage.

Trojan horse :

A Trojan horse is a program which seems to be doing one thing, but is actually doing another. A trojan horse can be used to set up a back door in a computer system such that the intruder can gain access later.

Virus

 A virus is a self-replicating program that spreads by inserting copies of itself into other executable code or documents. Therefore, a computer virus behaves in a way similar to a biological virus  which spreads by inserting itself into living cells.

 

Worm 

Like a virus, a worm is also a self-replicating program. A worm differs from a virus in that it propagates through computer networks without user intervention

Windows Shortcuts

 

    *

      Shift + F10 right-clicks.
    *

      Win + L (XP Only): Locks keyboard. Similar to Lock Workstation.
    *

      Win + F or F3: Open Find dialog. (All Files) F3 may not work in some applications which use F3 for their own find dialogs.
    *

      Win + Control + F: Open Find dialog. (Computers)
    *

      Win + U: Open Utility Manager.
    *

      Win + F1: Open Windows help.
    *

      Win + Pause: Open System Properties dialog.
    *

      Win + Tab: Cycle through taskbar buttons. Enter clicks, AppsKey or Shift + F10 right-clicks.
    *

      Win + Shift + Tab: Cycle through taskbar buttons in reverse.
    *

      Alt + Tab: Display Cool Switch. More commonly known as the AltTab dialog.
    *

      Alt + Shift + Tab: Display Cool Switch; go in reverse.
    *

      Alt + Escape: Send active window to the bottom of the z-order.
    *

      Alt + Shift + Escape: Activate the window at the bottom of the z-order.
    *

      Alt + F4: Close active window; or, if all windows are closed, open shutdown dialog.
    *

      Shift while a CD is loading: Bypass AutoPlay.
    *
    
      Ctrl + Alt + Delete or Ctrl + Alt + NumpadDel (Both NumLock states): Invoke the Task Manager or NT Security dialog.
    *

      Ctrl + Shift + Escape (2000/XP ) or (Ctrl + Alt + NumpadDot) : Invoke the task manager. On earlier OSes, acts like Ctrl + Escape.
    *

      Print screen: Copy screenshot of current screen to clipboard.
    *

      Alt + Print screen: Copy screenshot of current active window to clipboard.
    *

      Ctrl + Alt + Down Arrow: Invert screen. Untested on OS's other than XP.
    *

      Ctrl + Alt + Up Arrow: Undo inversion.
    *

      Win + B : Move focus to systray icons.
    *

      Ctrl + C or Ctrl + Insert: Copy.
    *

      Ctrl + X or Shift + Delete: Cut.
    *

      Ctrl + V or Shift + Insert: Paste/Move.
    *

      Ctrl + N: New... File, Tab, Entry, etc.
    *

      Ctrl + S: Save.
    *

      Ctrl + O: Open...
    *

      Ctrl + P: Print.
    *

      Ctrl + Z: Undo.
    *

      Ctrl + A: Select all.
    *

      Ctrl + F: Find...
    *

      Ctrl+W : to close the current window
    *

      Ctrl + F4: Close tab or child window.
    *

      F1: Open help.
    *

      F11: Toggle full screen mode.
    *

      Alt or F10: Activate menu bar.
    *

      Alt + Space: Display system menu. Same as clicking the icon on the titlebar.
    *

      Escape: Remove focus from current control/menu, or close dialog box.
    *

      Tab: Forward one item.
    *

      Shift + Tab: Backward one item.
    *

      Ctrl + Tab: Cycle through tabs/child windows.
    *

      Ctrl + Shift + Tab: Cycle backwards through tabs/child windows.
    *

      Enter: If a button's selected, click it, otherwise, click default button.
    *

      Space: Toggle items such as radio buttons or checkboxes.
    *

      Alt + (Letter): Activate item corresponding to (Letter). (Letter) is the underlined letter on the item's name.
    *

      Ctrl + Left: Move cursor to the beginning of previous word.
    *

      Ctrl + Right: Move cursor to the beginning of next word.
    *

      Ctrl + Up: Move cursor to beginning of previous paragraph. This and all subsequent Up/Down hotkeys in this section have only been known to work in    Rich Edit controls.
    *

      Ctrl + Down: Move cursor to beginning of next paragraph.
    *

      Shift + Left: Highlight one character to the left.
    *

      Shift + Right: Highlight one character to the right.
    *

      Shift + Up: Highlight from current cursor position, to one line up.
    *

      Shift + Down: Highlight from current cursor position, to one line down.
    *

      Ctrl + Shift + Left: Highlight to beginning of previous word.
    *

      Ctrl + Shift + Right: Highlight to beginning of next word.
    *

      Ctrl + Shift + Up: Highlight to beginning of previous paragraph.
    *

      Ctrl + Shift + Down: Highlight to beginning of next paragraph.
    *

      Home: Move cursor to top of a scrollable control.
    *

      End: Move cursor to bottom of a scrollable control.
    *

      Arrow Keys: Navigate.
    *

      Shift + Arrow Keys: Select multiple items.
    *

      Ctrl + Arrow Keys: Change focus without changing selection. "Focus" is the object that will run on Enter. Space toggles selection of the focused item.
    *

      (Letter): Select first found item that begins with (Letter).
    *

      BackSpace: Go up one level to the parent directory.
    *

      Alt + Left: Go back one folder.
    *

      Alt + Right: Go forward one folder.
    *

      Enter: Activate (Double-click) selected item(s).
    *

      Alt + Enter: View properties for selected item.
    *

      F2: Rename selected item(s).
    *

      Ctrl + NumpadPlus: In a Details view, resizes all columns to fit the longest item in each one.
    *

      Delete: Delete selected item(s).
    *

      Shift + Delete: Delete selected item(s); bypass Recycle Bin.
    *

      Ctrl while dragging item(s): Copy.
    *

      Ctrl + Shift while dragging item(s): Create shortcut(s).
    *

      In tree pane, if any:
    *

      Left: Collapse the current selection if expanded, or select the parent folder.
    *

      Right: Expand the current selection if collapsed, or select the first subfolder.
    *

      Numpad Asterisk: Expand currently selected directory and all subdirectories. No undo.
    *

      Numpad Plus: Expand currently selected directory.
    *

      Numpad Minus: Collapse currently selected directory.


    *

      Switch FilterKeys on and off. Right SHIFT for eight seconds
    *

      Switch High Contrast on and off. Left ALT +left SHIFT +PRINT SCREEN
    *

      Switch MouseKeys on and off. Left ALT +left SHIFT +NUM LOCK
    *

      Switch StickyKeys on and off. SHIFT five times
    *

      Switch ToggleKeys on and off. NUM LOCK for five seconds