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 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.