What is Subnet and Subnet Mask?

Subnet:A subnet (short for "subnetwork") is an identifiably separate part of an organization's network. Typically, a subnet may represent all the machines at one geographic location, in one building, or on the same local area network (LAN). Having an organization's network divided into subnets allows it to be connected to the Internet with a single shared network address. Without subnets, an organization could get multiple connections to the Internet, one for each of its physically separate subnetworks, but this would require an unnecessary use of the limited number of network numbers the Internet has to assign. It would also require that Internet routing tables on gateways outside the organization would need to know about and have to manage routing that could and should be handled within an organization.

A subnet is a also logical grouping of connected network devices. Nodes on a subnet tend to be located in close physical proximity to each other on a LAN. Network designers employ subnets as a way to partition networks into logical segments for greater ease of administration. When subnets are properly implemented, both the performance and security of networks can be improved.

Subnet Mask:
A mask used to determine what subnet an IP address belongs to. An IP address has two components, the network address and the host address. For example, consider the IP address 150.215.017.009. Assuming this is part of a Class B network, the first two numbers (150.215) represent the Class B network address, and the second two numbers (017.009) identify a particular host on this network.

Subnetting enables the network administrator to further divide the host part of the address into two or more subnets. In this case, a part of the host address is reserved to identify the particular subnet. This is easier to see if we show the IP address in binary format. The full address is:

10010110.11010111.00010001.00001001

The Class B network part is:

10010110.11010111

and the host address is

00010001.00001001

If this network is divided into 14 subnets, however, then the first 4 bits of the host address (0001) are reserved for identifying the subnet.

The subnet mask is the network address plus the bits reserved for identifying the subnetwork. (By convention, the bits for the network address are all set to 1, though it would also work if the bits were set exactly as in the network address.) In this case, therefore, the subnet mask would be 11111111.11111111.11110000.00000000. It's called a mask because it can be used to identify the subnet to which an IP address belongs by performing a bitwise AND operation on the mask and the IP address. The result is the subnetwork address:
Subnet Mask 255.255.240.000 11111111.11111111.11110000.00000000
IP Address 150.215.017.009 10010110.11010111.00010001.00001001
Subnet Address 150.215.016.000 10010110.11010111.00010000.00000000

The subnet address, therefore, is 150.215.016.000.

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Routing Tables

A routing table is an electronic document that stores the routes to the various nodes in a computer network. The nodes may be any kind of electronic device connected to the network. The Routing Table is usually stored in a router or networked computer in the form of a database or file. When data needs to be sent from one node to another on the network, the routing table is referred to in order to find the best possible route for the transfer of information.

Almost all computers and network devices connected to Internet use routing tables to compute the next hop for a packet. It is electronic table that is stored in a router or a networked computer. The routing table stores the routes (and in some cases, metrics associated with those routes) to particular network destinations. This information contains the topology of the network immediately around it. The construction of routing table is the primary goal of routing protocols and static routes.

Routing tables can generally be maintained manually when the network is small and static. The routing tables for all static network devices never change unless and until the administrator of the network changes them manually. In dynamic routing, the devices themselves automatically build and maintain their own routing tables. They do this by exchanging information regarding the network topology using routing protocols. This enables the devices in the network to automatically adapt to the changes in the network like device failures and network congestion as and when they occur.

Netware

The NetWare operating system was one of the first software products built for the networking of personal computers (PCs). NetWare emphasizes file and print serving capabilities, and the predominant use of NetWare is as a LAN server. Early version of NetWare appeared in the late 1980s, and today NetWare enjoys an installed base of millions of computers.NetWare is just one of many networking software products and services developed by Novell, Inc. Despite stiff competition from Microsoft and various other companies, Novell continues to offer strong network technology.The netware operating system is also known as Novell NetWare.

Know more about USB

USB is a high-performance serial bus communication technology. Most new computers and associated peripheral devices like printers and scanners contain built-in support for this technology. USB hubs for file and printer sharing also exist. USB and FireWire are the most popular, competing standards for networking computer peripherals.

Multiple versions of USB have been developed by the computer industry:

* USB 1.0 and 1.1: the first commercial versions supported a maximum data rate of 12 Mbps
* USB 2.0: the current version supports a much faster theoretical maximum rate of 480 Mbps
* USB 3.0: the future standard is expected to support up to 4.8 Gbps

Using USB for Local Networking
To build a USB network, simply connect USB cables to the USB ports on those devices. USB is plug and play compatible, meaning the operating system USB driver software automatically detects and configures device connections. One USB network supports up to 127 devices.

USB interfaces are backward compatible; for example, a USB 2.0 and a USB 1.1 can be networked. When two ends of a connection support different versions of USB, the link automatically runs at the lower speed of the older version.

TraceRoute command

Traceroute is a utility program that monitors the network path of test data sent to a remote computer. On Unix and Linux computers, the "traceroute" application is available in the shell, while on Windows computers, the "tracert" program can be accessed from DOS.Traceroute programs take the name or IP address of a remote computer on the command line. When run, traceroute sends a series test messages over the network (using ICMP) to each intermediate router progressing until the last message finally reaches its destination. When finished, traceroute displays the ordered list of routers that represent the path from that computer to the destination.

Understanding Protocol Suites

After knowing the importance of protocol in communication,designers have chosen to divide the communication problem into sub pieces and to design a protocol for each of the sub piece instead of having a single giant protocol that specifies complete details for all possible forms of communications.Doing so makes each protocol easier to design,analyze,implement and test.The main advantage of dividing communication software into multiple protocols increases flexibility because it allows subsets of protocols to be used as added.The division into separate protocols must be chosen carefully to ensure the resulting communication system is efficient and effective.To avoid duplication of effort,each protocol should handle part of the communication problem not handled by other protocols.

To make efficient implementation possible,protocols should be designed so that,they can share data structures and information.Finally,the combination of protocols should handle all possible hardware failures or other exceptional conditions.Now the main question that arises in front of us is that how can one guarantee that protocols will work together.The answer of this question lies in an overall design plan that means instead of developing each protocol in isolation,protocols are designed and developed in complete,cooperative sets called suites or families.Each protocol in a suite solves one part of the communication problem;together they solve the entire communication problem.Furthermore,the entire suite is designed to make interactions among protocol efficient.

Need for Protocols

As all we know that basic communication hardware consists of mechanisms that can transfer bits from one point to another.However,using raw hardware to communicate is analogous to programming by entering 1s and 0s but it is cumbersome and inconvenient.To aid programmers,computers attached to a network use complex software that provides a convenient,high level interface for applications.The software handles most low level communication details and problems automatically,making it possible for applications to communicate easily.Thus,most application programs rely on network software to communicate that means hey do not interact with network hardware directly.All parties involved in a communication must agree on a set of rules to be used when exchanging messages and such type of agreement is known as protocol.This term is also applied to computer communications meaning of which "a set of rules that specify the format of messages and the appropriate action required for each message is known as a network protocol or a computer communication protocol."The software that implement such rules is called as protocol software.

Significance of Internetworking and TCP/IP

Internetworking has become one of the most important ideas in modern networking.In fact,Internet Technology has revolutionized computer communication.Most large organizations already use internetworking as the primary computer communication mechanism.Smaller organizations and individuals are beginning to do so well.More important,in addition to private internet,the TCP/IP technology has made possible a global internet that reaches schools,commercial organizations,and government and military sites in all populated countries around the world.The worldwide demand for internetworking products has affected most companies that sell networking technologies.

Competition has increased because new companies have been formed to sell hardware and software need for internetworking.In addition may companies have modified their protocol designs to accommodate internetworking.In particular,most network protocols were originally designed to work with one network technology and one physical network at a time.To provide internetworking capabilities,companies have extended the designs in two ways:the protocols have been adapted to work with many network technologies and new features have been added that allows the protocols to transfer data across internet.

Combining Switches and Hubs

From the last post it is clear that switching provides higher aggregate data rates in comparison to a hub,but the main drawback that occurred with the switched networks is the higher cost in comparison to hub.To overcome all these problems network administrator choose a compromise between the hub and the switched networks that means instead of connecting one computer to each port on a switch,the administrator connects the hub to each of the port of the switch and then connect each computer to one of the hubs.This result is very much closer to the conventional bridged LAN where each hub appears to be a single LAN segment, and the switch makes it appear that bridges connect all segments.The system also performs like a conventional bridged LAN where a computer must share a bandwidth with other computers connected to the same hub,communication can occur in parallel between a pair of computers attached to one hub and a pair of computers attached to another.

Concept of Switching

In general,a network technology is called switched if the hardware includes an electronic device that connects to one or more computers and allows them to send and receive data from each other.More specifically,a switched LAN consisted of a single electronic device that transfers frames among many computers.As all we know that a switch resembles a hub,like a hub, a switch consists of a single box with multiple ports that each attach to a single computer.The main difference between a switch and a hub arises from the way the devices operates,a hub simulates a single shared medium while in the case of a switch it simulates a bridged LAN with one computer per segment.

If we talk about the chief advantage of using a switched LAN instead of a hub then the reason is same as that of using bridged LAN instead of a single segment.In the case of a hub a single segment is shared by all computers,that means at most two computers can communicate through hub at a time.Thus the maximum possible throughput of a hub system is R,the rate at which single computer can send data across a LAN segment.In the case of a switched LAN,each computer has a simulated LAN segment to itself that means the segment is busy only when a frame is being transferred to or from the computer.As a result,as many as one half of the computers connected to a switch can send the data at the same time(if they send to one of the computers that is not busy in sending).Thus,in this case the maximum throughput of a switch is RN/2 where R is the rate at which data transmits,and N is total number of computers that are connected to the switch.

Understanding Baud Rate and Framing Errors

As all we know that sending and receiving hardware are agreed on the length of the time the voltage will be held for each bit.Instead of specifying the time per bit,which is a small fraction of second,communication systems specify the number of bits that can be transferred in a second.For ex:some early RS-232 connections operated at 300 bits per second,19200 bits per second and 33600 bits per second are more common.Technically ,transmission hardware is rated in baud,the number of changes in the signal per second that the hardware generated.For the simple RS-232 scheme presented, the baud rate is exactly equal to the number of bits per second.Thus,9600 baud means 9600 bits per second.

To make RS-232 hardware more general,manufacturers usually design each piece of hardware to operate at variety of baud rates.The baud rate can be configured either manually by physically setting switches on the hardware when it is installed in a computer or automatically by device driver software in a computer.If the sending and receiving hardware are not configured to use the same baud rate,errors will occur because the receiver's timer will not wait an appropriate length of time for each bit.To detect errors,a receiver's measure the voltage for each bit multiple times and compares the measurements.If the voltage do not all agree or if the stop bit does not occur exactly at the time expected,the receiver reports an error.Such errors are known as framing errors.

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.

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.

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.

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).

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.

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 220.123.152.1 and hardware address 1. The second host is assigned an IP address 220.123.152.2 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.

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.

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.

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.