Tuesday, December 23, 2008

Concept of Parity Bits

Fortunately,few communication systems experience frequent experience.More important,the interference on a long distance communication circuit may be so small that modems can handle all problem automatically.Despite the low probability of error,scientists and engineers who design networks understand that transmission errors do occur,and provide hardware and software mechanisms to detect and correct such problems.But now a days most RS-232 circuits use the different mechanism to ensure that each character arrives intact.This mechanism is known as Parity Check mechanism,this mechanism requires the sender to compute an additional bit,called a parity bit,and to attach it to each character before sending.After all bits of a character arrives,the receiver removes the parity bit,performs the same computation as the sender,and verifies that result agrees with the value of the parity bits.The parity computation chosen is such that if one of the bits in the character is damaged in transit,the receiver's computation will not agree with the parity bit and the receiver will report that an error occurred.

There are two forms of parity,even or odd.Both the sender and receiver must agree on which form is being used.In either form,the computation of the parity bit for a given character is straightforward.To achieve even parity,the sender sets the parity bit to 0 or 1 whichever will make the total number of 1 bits(including parity bit)an even number.Thus,when using even parity ,the parity bit for 0100101 is 1 because the character contains an odd number of 1 bits,and the parity bit for 0101101 is 0 because the character already contains an even number of 1 bits.Similarly to achieve odd parity,the sender chooses a parity bit such that it makes the total number of one in the character is odd.If all bits from the character arrive intact,the receiver's computation of parity will agree with the sender's.If interference changes one of the bits during transmission,the receiver's computation will not agree with the sender's and the receiver will report a parity error.

Sunday, December 21, 2008

Transmission errors in Computer Networks

Lightning,Power Surges,and other electro-magnetic interference can introduce unwanted electrical currents in the electronic components or wires used for communication.Interference that is severe especially lightning can cause permanent damage to network equipment.More often,however interference merely changes the signal used for transmission without damaging the equipment.A small change in the electrical signal can cause the receiver to misinterpret one or more bits of the data.In fact,interference can completely destroy a signal,meaning that although the sender transmits,the receiver does not detect that any data has arrived.Surprisingly,interference on a completely idle transmission circuit can create the opposite effect that means the sender does not transmit any thing,a receiver might interpret the incoming interference as a valid sequence of bits or characters.All these types of errors are known as transmission errors.In the last we can summarize the whole thing in this way that much of the complexity in the computer networks arises because digital transmission system are susceptible to interference that can cause random data to appear or transmitted data to be lost or changed.

Saturday, December 20, 2008

Concept of Packets

Most computer networks do not transfer data as an arbitrary string of continuous bits.Instead the network system divides the data into small blocks called packets,which it sends individually.Computer networks are often called packet networks or packet switching networks because they use packet technology.The concept of using data packets for data transmission is supported by two facts,first a sender and receiver need to coordinate transmission to ensure that data arrives correctly.Dividing the whole data into small blocks helps a sender and receiver to determine which blocks arrive correctly and which blocks do not.

The second fact that supports packet technique is as all we know that communication circuits and modem hardware are expensive,multiple computers often shares underlying connection and hardware.To ensure that all computers receive fair,prompts access to a shared communication facility,a network system cannot allow one computer to access deny to other computers.Using small packets helps ensure fairness.Let understand this more clearly.The networks that do not packet technology allows an application program(computer) to hold a shared communication resource arbitrarily long that means an application was permitted to finish before another application could begin using the resource.But now the networks that uses packet technology have a solution of this problem.Here the networks permits one computer to send a packet of data and then blocks that computer from sending another packet.Meanwhile the network permits another computer to send a packet and so on.A single computer can hold a shared resource only long enough to send single packet,and must wait until other computers have a turn before sending a second packet.

Friday, October 17, 2008

Understanding Proxy Server

Proxy Server:A proxy server is a kind of buffer between your computer and the Internet resources you are accessing. The data you request come to the proxy first, and only then it transmits the data to you.Proxy servers accumulate and save files that are most often requested by thousands of Internet users in a special database, called “cache”. Therefore, proxy servers are able to increase the speed of your connection to the Internet. The cache of a proxy server may already contain information you need by the time of your request, making it possible for the proxy to deliver it immediately.Security and privacy. Anonymous proxy servers that hide your IP address thereby saving you from vulnerabilities concerned with it. Sometimes you may encounter problems while accessing to web server when server administrator restricted access from your IP or even from wide IP range (for example restricting access from certain countries or geographical regions). So you try to access those pages using an anonymous proxy server.

Public Proxy Server:It is a proxy server which is free and open for everybody on the Internet. Unfortunately most of them are not anonymous.Free service trying to provide list of public HTTP proxy servers. Usually provide small list of proxies with low percent of functioning servers due to hosting restrictions on CPU time (they simply can't allow themselves to check many proxies every second especially in parallel).

Monday, October 13, 2008

Address Binding Technique Part-3

Now as all we aware of the first and second technique of address binding, now its high time to tell you the third and last technique of address binding.In this technique the address resolution takes place through message exchange.

Address Resolution with message exchange: In this approach the computer that needs to resolve an address sends a message across network and receives reply.The message carries a request that specifies the protocol address,and the reply carries the corresponding hardware address.Now the main question that came in front of us is that where all these requests be sent.Generally there are two designs that are used for this.In the first design a network includes one or more servers that are assigned the task of answering address resolution requests.

Whenever,address resolution is needed a message must be sent to any of these servers which will send a reply.In the second design no special address resolution servers are needed.Instead,each computer on the network participates in address resolution by agreeing to answer resolution requests for its address.When a computer needs to resolve an address,it broadcasts a request on the network.All machine receive the request and examine the requested address.If an incoming request matches a computer’s address, the computer responds.Now discuss the advantages of both the designs.

The chief advantage of the first scheme arise from centralization because a few address resolution servers handle all resolution tasks on the network,address resolution is easier to configure,manage and control.Now the advantage of second design arise from distributed computation.Address resolution servers may be expensive.In addition to the expense of additional hardwares like extra memory.In addition to this servers are expensive to maintain because address binding information stored in servers must be updated whenever new computers are added to the network or hardware address changes.Furthermore ,address resolution servers can become a bottleneck on a large busy network.If each computer is able to resolve its own address ,then there is no need of servers.

Wednesday, October 8, 2008

Address Binding Technique 2

Hello friends in the last post I told you about the first technique through which address binding takes place.Now in this post I am going to tell you second technique through which address binding takes place.

Address Resolution with closed form computation: As all we know that many network technologies use static physical addresses, some technologies use configurable addressing in which a network interface can be assigned a specific hardware address. For such type of networks it is possible that we can choose addresses that make closed form address resolution possible.A resolver that uses a closed form method computes a mathematical function that maps an IP address to its equivalent hardware address. If the relationship between the IP address and its coprresponding hardware address is straightforward, the computation requires only a few arithmetic operations.Infact,the host portion of a computer’s IP address can be chosen such that it is identical to the computer’s hardware address.

Let’s understand this situation with the help of a example, suppose a configurable network has been assigned the network address 220.123.152.xxx and its suffix ranging from 0-24. As computers are added to the network, each computer is assigned an IP address from this range and a matching hardware address.The first host is assigned an IP address and hardware address 1. The second host is assigned an IP address and hardware address 2. The suffixes need not be sequential, if a router attached to a network assigned IP address202.123.152.101,then the router is assigned an IP address of 121.Given the IP address of any computer on the network, the computer’s hardware address can be computed by a single Boolean ‘and’ operation.

Hardware Address=IP Address & 0xff

As I think it is clear from the example that why closed form resolution is often used with configurable networks.

Tuesday, October 7, 2008

Address Binding Technique 1

Hello friends in this post I am going to tell you all the first way through which address binding takes place.

1.Address resolution with table lookup: This technique of address resolution requires a data structure that contains information about address binding. The table consists of an array. Each entry in the array contains a pair (P,H),where P is the protocol address and H is its corresponding hardware address. The main advantage of table lookup approach is generality- a table can store the address bindings for an arbitrary set of computers on a given network. In particular a particular protocol address can map to an arbitrary hardware address.

Furthermore, the table lookup algorithm is straight forward and easies among all the approaches. Suppose we have given a next-hop address, N, the software searches the table until it finds an entry where the IP address matches N. The software then extracts the hardware address from the entry.For a network that contains a less then dozen of hosts, a sequential search is sufficient ,the resolution software begins at first entry and searches each entry in table until a match is found. But for large networks this sequential search is not possible because it consumes excessive CPU time. In these type of situations to improve computational efficiency we can use Hashing or Direct Indexing.

As all we know that Hashing is a general purpose data structure and is well known to all the programmers, so, here I am not going to discuss it. Let’s take a look at the other technique that is of Direct Indexing. Direct Indexing is slightly more efficient but less general technique. In particular Direct Indexing is possible only in those cases where protocol addresses are assigned from a compact range.

For ex: Direct Indexing can be used with IP addresses that are arranged in sequential order. In these type of cases the software maintains a one dimensional array of hardware addresses, and uses the host suffix from an IP address as an index to an array.

In the next post I will tell you the second technique of Address Binding.

Sunday, October 5, 2008

Address Resolution Protocol

Hello friends in this post I am going to discuss one of the most interesting topics in computer networks that is of Binding Protocol Addresses or Address Resolution Protocol.

As all we know that IP addresses are virtual because they are maintained by software. Neither Local Area network nor Wide Area Network hardware understands the relationship between an IP address prefix and a network or the relationship between an IP address suffix and a particular computer. I am telling you all these things because we have to know all these things before understanding Binding Address Protocol. Now let’s come to our main issue of Binding Protocol Addresses, the first point that came into the mind of any one that is new to networking is what Binding Protocol Address is and what its requirement is.

Now before giving answer of first question I would like to give the answer of our second question. Let’s consider an application program that generates data to be sent across an internet, for this software places the data in the packet, which contains the protocol address (IP address) of the destination. Software in each host or router uses the protocol destination address to select the next hop for the packet. Once a next hop has been selected, software transfers the packet across one physical network to the selected host or router. To provide the illusion of a single, large network, software works with the IP addresses when forwarding the packets. Both the next hop and destination addresses are the IP addresses.

Now the main problem arises here, when we want to transmit frames across physical network hardware we need hardware address of the destination because the hardware does not understand the IP addressing. So, when a frame is set across a physical network it must use the hardware’s frame format and all the addresses in the frame must be hardware addresses. Consequently, the protocol address of the next hop must be translated to an equivalent hardware address before a frame can be sent. The process of converting IP address to its equivalent hardware address is known as Address Resolution and a protocol address is said to be resolved to the correct hardware address. Address Resolution is local to network that means one computer can resolve the address of another computer only if both computers attach to the same physical network. A computer never resolves the address of a computer on a remote network.

After this explanation I hope you understand a little bit about the Address Resolution Protocol. In the next post I will tell about the various techniques through which Address Resolution takes place. Hope you like this post.If you have any doubts in any of the topics of Computer Networks then post your problem in the comment. I will try to sort it out.

Wednesday, October 1, 2008

Types of Viruses

Hello friends in this post i am going to give you a whole detail of almost all of the virus types in which trojan is the most common one.

Trojan:Remember the Trojan Horse? Bad guys hid inside it until they could get into the city to do their evil deed. A trojan computer program is similar. It is a program hitch does an unauthorized function, hidden inside an authorized program. It does something other than what it claims to do, usually something malicious (although not necessarily!),and it is intended by the author to do whatever it does. If it's not intentional, its called a 'bug' or, in some cases, a feature :) Some virus scanning programs detect some trojans. Some virus scanning programs don't detect any trojans. No virus scanners detect all trojans.

Virus: A virus is an independent program which reproduces itself. It may attach to other programs, it may create copies of itself (as in companion viruses). It may damage or corrupt data, change data, or degrade the performance of your system by utilizing resources such as memory or disk space. Some virus scanners detect some viruses. No virus scanners detect all viruses. No virus scanner can protect against "any and all viruses, known and unknown, now and forevermore".

Worm:Made famous by Robert Morris, Jr. , worms are programs which reproduce by copying themselves over and over, system to system, using up resources and sometimes slowing down the systems. They are self contained and use the networks to spread, in much the same way viruses use files to spread. Some people say the solution to viruses and worms is to just not have any files or networks. They are probably correct.We would include computers.
Logic Bomb:Code which will trigger a particular form of 'attack' when a designated condition is met. For instance, a logic bomb could delete all files on Dec. 5th. Unlike a virus, a logic bomb does not make copies of itself.

Hope you found this post useful.Waiting for your responses.

DHCP Server

Hello friends in this post i am giving answer to one of the most common question from the field of networking,that is how an IP address is assigned to any computer or any device like switch,hub,bridge.

DHCP is the only responsible to assign IP address to any device.Dynamic Host Configuration Protocol (DHCP) is a network protocol that enables a server to automatically assign an IP address to an individual computer's TCP/IP stack software. DHCP assigns a number dynamically from a defined range of numbers configured for a given network.Today you found almost all wireless access points, many wired Ethernet routers, and computers running Internet Connection Sharing have built-in DHCP servers.Due to this reason DHCP servers are mainly preffered for the small networks in comparison to large networks.DHCP assigns a TCP/IP address when a system is started. Typically, it works like this:
1. A user turns on a computer with a DHCP client.

2. The client computer sends a broadcast request (called a DISCOVER or DHCPDISCOVER), looking for a DHCP server to answer.

3. The router directs the DISCOVER packet to the correct DHCP server.

4. The server receives the DISCOVER packet. Based on availability and usage policies set on the server, the server determines an appropriate address (if any) to give to the client. The server then temporarily reserves that address for the client and sends back to the client an OFFER (or DHCPOFFER) packet, with that address information. The server also configures the client's DNS servers, WINS servers, NTP servers, and sometimes other services as well.

5. The client sends a REQUEST (or DHCPREQUEST) packet, letting the server know that it intends to use the address.

6. The server sends an ACK (or DHCPACK) packet, confirming that the client has a been given a lease on the address for a server-specified period of time.

When we assign IP address statically to any computer then there is always a probability that two computers are configured with the same IP address. This creates a conflict that results in loss of service. Using DHCP to dynamically assign IP addresses minimizes these conflicts.

Tuesday, September 30, 2008

Classful Vs Classless Address

Hello friends in this post i am again going to tell you some more interesting concepts of networking.In this post i am going to tell you what is the difference between classfull and classless address.
Classful addressing:In the classful addressing system all the IP addresses that are available are divided into the five classes A,B,C,D and E, in which class A,B and C address are frequently used because class D is for Multicast and is rarely used and class E is reserved and is not currently used.Each of the IP address belongs to a particular class that's why they are classful addresses.Earlier this addressing system did not have any name,but when classless addressing system came into existence then it is named as Classful addressing system.The main disadvantage of classful addressing is that it limited the flexibility and number of addresses that can be assigned to any device.One of the major disadvantage of classful addressing is that it does not send subnet information but it will send the complete network address. The router will supply its own subnet mask based on its locally configured subnets. As long as you have the same subnet mask and the network is contiguous, you can use subnets of a classful network address.

For more details , please visit here and here.

Classless Addressing:Classless addressing system is also known as CIDR(Classless Inter-Domain Routing).Classless addressing is a way to allocate and specify the Internet addresses used in inter-domain routing more flexibly than with the original system of Internet Protocol (IP) address classes.What happened in classful addressing is that if any company needs more than 254 host machines but far fewer than the 65,533 host addresses then the only option for the company is to take the class B address.Now suppose company needs only 1000 IP addresses for its host computers then in this (65533-1000=64533) IP addresses get wasted.For this reason, the Internet was, until the arrival of CIDR, running out of address space much more quickly than necessary. CIDR effectively solved the problem by providing a new and more flexible way to specify network addresses in routers.A CIDR network address looks like this:

For more details , please visit here and here.

The "" is the network address itself and the "15" says that the first 15 bits are the network part of the address, leaving the last 17 bits for specific host addresses.One more advantage of classless addressing is that classless protocol sends subnet information. This allows you to create discontiguous networks with any given classful network address.

 For more details , please visit here and here.

Sunday, September 28, 2008

Repeater Vs Switch Vs Hub

Hello friends this is my post and i this post i am going to give you a brief description and difference between various networking devices.

As all we know various types of devices like repeaters,hub, routers, switch are used very frequently in the setup of any computer network.Here in this post we see all these devices one by one.

Repeater: A repeater is an electronic device that is used to receive a signal and after receiving, it transmit signal to the higher level so that it can move to long distance without any destruction in its value.In other words we can also say that repeater is a device that is designed to overcome signal attenuation means to recover signal degradation.All repeaters has two ports one for receiving the signal and other for its transmission. Repeaters can be used for carrying electrical signals in the form of copper wire or to carry light in the form of fiber optics.It is a layer 1 device that means Physical layer device.

Hub: In simple words we can say that a hub is a multi port repeater.It means it allows multiple machines to connect to the same network, by broadcasting traffic to all of its ports.The most practical implication of this is that only one device can send data at a time, otherwise a collision occurs. Hubs implement a collision detection multiple access method to detect when two machines attempt to send data at the same time. A hub supports broadcasting of any signal.It is also layer 1 device.

Switch:In general switch is very much different from the repeater and the hub , both of them works on IP address while the switch works on the MAC address. A switch works by keeping track of which MAC addresses are attached to each port. It inspects traffic and only sends it to the port that the destination device is on (based on the destination MAC address). A switch had a finite amount of memory in which it can store MAC addresses, smaller switches such as ones intended for desktop use have less memory then larger ones that may form part of an enterprise networks backbone. When a switches memory becomes full of active MAC addresses it will often revert to functioning like a hub and broadcast traffic to all ports.A switch is a layer 2 device(Data link Layer).

Introducing Blog

Hello friends this is my first blog and in this blog i am going to explore all the important concepts of Computer Networks in which students find themselves in difficulty.Here you will find a complete description of all the topics that will really help you in understanding these concepts.In this blog you will also found some latest hacking technologies in Windows XP,UNIX and Linux that will definitely a fun stuff for you.In addition to this i also give you latest news from the field of technology and also some reviews.