Disadvantages of a network

We List below some of the Disadvantages of a networking

• Crashes.  The biggest disadvantage is on a server based network. When the server crashes, work gets disrupted as all network resources and its benefits are lost. Unless proper precautions are taken to ensure regular backups, the lost may result in loss of days and even months of critical data and time.
• Data Security. As all data resources are all pooled together in a  network, it is possible for unauthorised personnel to access classified information if network security is weak or poorly implemented.
• Privacy. A network may also mean loss of privacy as anyone, especially your boss, with the right network privileges may be in a position to read your private e-mail or hook into your private chat session., thus breaching the privacy.
• There are advantages and disadvantages of sharing a program like Microsoft Word. On the plus side it makes easier to keep Microsoft Office updated, because if you want to upgrade the software you need to update only one copy of office, the one which is located on the server. Its easier to configure as well as a network version of an application is less expensive than if you copy a new application on each computer system.
• On the minus side, with so many users using a network to access a Program on the network, the network may get jammed, and the overall performance is poor.

Above were some of the disadvantages of setting up a computer network. But overall, the advantages of a networking are more than the disadvantages.

Related Posts

• Advantages of a Networking
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Advantages of networking

Here are some of the benifits of setting up a network

• Sharing Files : Networks let you share information with other computers on the network. Depending on how you set your network, you an do this in one of two ways. The most direct way is to send a file directly from your computer to your friend's computer. The second way is to send your file to an intermediate resting place, where your friend can pick it up later, kind of like dropping a bag full of ransom money at a phone booth. A third way is to permanently store the file at that intermediate place, where both of you can get at the file whenever you want. One way or the other, the data travels to your friend's computer over the network cable.
• Sharing Resources : This means that you can set up certain computer resources - like a disk drive or a printer - so that all of the computers on the network can access the. For example, the laser printer attached to server computer is a shared resource. That means that anyone on the network can use it. Without the network, the other computer members will have to buy their own laser printers.
• Disk drives can be shared resources too. In fact, a disk drive must be set up as a shared resource in order to share files with other users. Suppose in a Network a Computer number 3 wants to share a file with the computer number-4, and a shared disk drive has been set up at computer number 2. All computer number 3 has to do is to copy its file to the shared drive on number 2 computer and tell the number 4 computer where the file is put. Then when the number 4 comes around, he can copy the file from number 2 to its own computer.
• You can share other resources too, such as  CD-ROM devices (those new dangled devices that store gigabytes of data and are more useful to large clip-art libraries and encyclopedia) or modems (which let you access other computers that aren't on your network).
• A network allows users to share resources with others on the network. This can be hardware related like a printer or a modem, or it can be software related like programs and databases, or work processors. System administrators in a server based network can configure or install new software on network computers remotely.
• Sharing Programs. Sometimes its best to put programs that everybody uses on a shared disk on a computer, rather than keep separate copies of the programs on each person's computer. For example if you have 10 computer users who all uses Microsoft Word, you could either store Ten copies of a Microsoft Word on each computer or you can store a single copy on a central server computer in a network which each user can access. This also allows to just invest on one computer, the server, more than on other computers, resulting in overall lowering of cost of a network.
• Backups. As all data is stored on a server , backing up critical data is a simple process.
• Communication. The biggest benefit however, comes in the form of better communication-electronic mail and groupware applications. Through electronic mail or e-mail, members of a network can send messages and ensure safe delivery of data to other members, even when they are not there. Groupware applications allow users to work collectively on same document.

Above were some of the advantages of a network. This is by no means a comprehensive or a complete list, but just enough to give you an idea, on how useful it is to set up a computer network.

• Network Basics
• Disadvantages of a network.

Networking : Basics of computer Network

A network is nothing more than two or more computers conneted together by a cable so that they can exchange information. WIth a computer network, all the computers in the office are hooked together with cables, install a special network adapter card (an electronic circuit card that goes inside your computer-ouch!) in each computer so you'll have a place to plug in the cable, set up and configure special network software to make the network work, and viola, you have a working network,

node : Every computer connected to the network is said to be on the network. The technical term for the computer that is on network is node.

online and offline : When a computer is turned on, using computer on and off button, and is able to access the network the computer is said to be online. On the other hand if a computer is unable to access the network despite connectd to it, is called offline. A computer can be offline for several reasons, it could be unplugged, it could be broken, the cable that connects computer to network is damaged, or there could be wad of gum jammed into the disk drive.

computer network : In general terms a computer network is an interconnection of various computer systems located at the same/different places. The interconnection is done through a communication link also known as the physical layer of the network and the link is such that it is totally transparent to the users of the network. The transparency of the communication link is brought about by the interfacing software known as protocols which enables a user in one location to freely access a computer system/database/process/software in another location.

However, if the computers in the network operate together as a single unit which to user appears as a single computer, albeit physically dispersed, then the complete system is more accurately described as a distributed system. Therefore, although, any interconnected set of computers is often conveniently referred to as "computer network", the use of the term often implies as interconected set of independent computers and not a distributed system. However, it may be useful when considering a distributed system to be able to recognize the particular type of network on which it is based.

Computer networks are collection of computers and intelligent periperal equipment (e.g., automatic teller machines and point-of-sale terminals equiped with microprocessors) that are inteconnected by telephone lines, microwave relays, and other high speed communication links for the purpose of exchanging data and sharing equipment. Networking has been developed on all levels, from local to international, in diverse sectors of society. Examples are networks used by government organizations for rapid retrieval of information form databases at central locations and those used by banks and retail merchants for the transfer of funds or credit verification. If telephone lines adopted for a computer network, each computer in the network must be connected to a telephone line via a ,modem (modulator/demodulator)

Modem  is a device which converts digital pulses from a computer into analog signals to be transmitted over telephone lines and vice versa. When a machine in a network communicates with another computer, both follow a predetermined potocol (i.e set of procedures), according to which data are exchanged.

LAN A local-area network (LAN)  is a communication network consisting of many computers (mostly personal computers and workstations) that are placed within a local area, such as single building or company.

Servers: Since mid-1980's , LANs have become widespread becuse personal computers and workstations are used extensively in offices, enabling individual users to run their specific programs easily and to have their own files at hand on floppy disks. Individual users can also share data or files on LAN as if the data or file reside on their respective computers; computers that are designed or used for this purpose are called servers.

Laser Printers and other peripheral rquipment also can be connected to a network for common use. Coaxial cables or fibre optic cables are popular communication lines because they provide fast data transmission and are easy to install within buildings. In the case of LANs, there are a few different protocols, such as Ethernet and Token Ring, developed by Xerox and IBM, respectively.

Network Components
The key network components and alternatives are:

• Media: Twisted pair wire, coaxial cable, fibreoptics, Microwave, satellites, cellular radio, point to point transreceivers.
• Processors: Modems, Multiplexers, concentrators, routers, bridges, gateways, Front-end processors, client and server computers.
• Software: Communication software, Network operating system Netware, Slip or PPP, POP, smtp, MSN internet explorer or Netscape Navigator, middleware.
• Channels: Analog/digital, switched/non-switched, circuit/message/packet switching, simplex/duplex, synchronous/ asynchronous, speed and baud rate.
• Topology: Point-to-Point, multidrop, star, ring, mesh, Ethernet
• Architecture: OSI, IEEE, ISDN, PSTN

Frankly, computer networks are a bit of pain to set up. So why bother? Because the benifits of having a network make the pain of setting one up bearable.

Related Posts:

• Advantages of Networking
• Disadvantages of a Network
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Internet Summary- Internet in a Nutshell (Fact sheet / Cheatsheet)

Following fact sheet provides quick summary about Internet:

• The Internet is a collection of more than 100,000 separate networks.
• ARPANET (the early internet) began as a network with four nodes.
• TCP/IP is the protocol suite for the Internet.
• CSNET provided communication between networks ineligible to join ARPANET.
• NSFNET provided communication between networks throughout the United States.
• Local Internet Service Providers (ISPs) connect individual users to the Internet.
• Regional Internet service providers connect local Internet service providers.
• National service providers (NSPs) are backbone networks created and maintained by specialized companies.
• A protocol is a set of rules that governs data communications; the key elements of a protocol as syntax, semantics and timing.
• Standards are necessary to ensure that products from diffrent manufacturers can work together as expected.
• The ISO, ITU-T, ANSI, IEEE, and EIA are some of the orgaizations involved in standards creation.
• Forums are special-interest groups that quickly evaluate and standardize new technologies.
• Two important forums are Frame Relay Forum and ATM Forum.
• The FCC is a regulatory agency that regulates radio, television, and wire/cable communications.
• A Request For Comment (RFC) is an idea or concept that is precursor to an Internet Standard.
• An RFC is categorized as required, recommended, elective, limited use, or not recommended.
• The Internet Society (ISOC) promotes research and other scholarly activities related to the Internet.
• The Internet Architecture Board (IAB) is the technical advisor to the ISOC.
• The Internet Engineering Task Force (IETF) is a forum of working groups responsible for identifying operational problems and proposing solutions to these problems.
• The Internet Research Task Force (IRTF) is a form of working groups focusing on long-term research topics related to Internet protocols, applications, architecture, topology , and technology.
• The Internet Corporation for Assigned Names and Numbers (ICANN), formerly known as IANA, is responsible for the management of Internet domain names and addresses.
• The Network Information Center (NIC) is responsible for collecting and distributing information about TCP/IP protocols.

Above were Important Facts about Internet, The Cheat sheet showed about The Organisation and people who monitor and administers Internet, as well as about how Internet came into existence and evolved from ARPANET.

Internet Administration-Who Owns It

Nobody owns Internet. The Internet, with its roots primarily in the research domain, has evolved and gained a broader user base with significant commercial activity. Various groups taht coordinate Internet issues have guided this growth and development.

• Internet Society (ISOC). http://www.isoc.org/ The Internet Society is an international, non profit organization formed in 1992 to provide support for the Internet standards process. ISOC accomplishes this through maintaining and supporting other Internet administrative bodies such as IAB, IETF, IRTF, and IANA. ISOC also promotes research and other scholarly activities relating to the Internet.
• Internet Architecture Board (IAB). http://www.iab.org/  The Internet Architecture Board is the technical advisor ISOC. The main purposes of the IAB are to oversee the continuous development of the TCP/IP Protocol Suite and to serve in technical advisory capacity to research members of the Internet community. IAB accomplishes this through its two primary components, the Internet Engineering Task Force (IETF) and the Internet Research Task force (IRTF). Another responsibility of the IAB is the editorial management of the RFCs. IAB is also the external liaison between the Internet and other standards organizations and forums.
• Internet Engineering Task Force (IETF). http://www.ietf.org/ The Internet Engineering Task Force is a forum of working groups managed by the Internet Engineering Steering Group (IESG). IETF is responsible for identifying operational problems and proposing solutions to these problems. IETF also develops and reviews specifications intended as Internet standards. The working groups are collected into areas, and each area concentrates on a specific topic. Currently nine areas have been defined, although this is by no means hard and fast number. The areas are: Applications, Internet Protocols, Routing, Operations, User Services, Network Management, Transport, Internet protocol next generation (IPng), and Security
• Internet Research Task Force (IRTF). http://www.irtf.org/  The Internet Research Task Force (IRTF) is a forum of working groups managed by the Internet Research Steering Group (IRSG). IRTF focuses on long-term research topics related to Internet protocols, applications, architecture, and technology.
• Internet Assigned Numbers Authority (IANA) and Internet Corporation for Assigned Names and Numbers (ICANN). http://www.icann.org/  The Internet Assigned Numbers Authority (IANA), supported by the U.S. government, was responsible for the management of Internet domain names and addresses until October 1998. At that time the Internet Corporation for Assigned Names and Numbers (ICANN), aprivate nonprofit corporation managed by an international board, assumed IANA operations.
• Network Information Center (NIC). The NIC is responsible for collecting and distributing information about TCP/IP protocols.

Above were the main organisations that administers Internet.

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• Internet Introduction and Overview.
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Internet Standards, RFC and Maturity Levels

An Internet Standard  is as thoroughly tested specification that is useful to and adhered to by those who work with the Internet. It is a formalized regulation that must be followed. There is a strict procedure by which a specification attains Internet standard status. A specification begins as an Internet draft. An Internet draft is a working document ( a work in progress) with no official status and a six-month lifetime. Upon recommendation from the Internet authorities, a draft may be published as a Request for Comment (RFC). Each RFC is edited, assigned a number, and made available to all interested parties.

RFCs go through maturity levels and are categorized according to their requirement level.

Maturity Levels
An RFC, during its lifetime, falls into one of six maturity levels: proposed standard, draft standard, Internet standard, historic, experimental, and Informational.

• Proposed Standard. A proposed standard is a specification that is stable, well understood, and of sufficient interest to the internet community.  At this level, the specification is usually tested and implemented by several different programs.
• Draft Standard. A proposed standard is elevated to draft standard status after atleast two successful independent and interoperable implementations. Barring difficulties, a draft standard, with modifications if specific problems are encountered, normally becomes an internet standard.
• Internet Standard. A draft standard reaches Internet standard after demonstrations of successful implementation.
• Historic. The Historic RFCs are significant from a historical perspective. They either have been superseded by later specifications or have never passed the necessary maturity levels to become an internet standard.
• Experimental. An RFC classified as experimental describes work related to an experimental situation that does not affect the operation of the internet. Such an RFC should not be implemented in any functional Internet service.
• Informational. An RFC classified as informational contains general, historical, or tutorial information related to the Internet. It is usually written by someone in a non-Internet organization, such as a vendor.

RFC Requirement Levels
RFCs are classified into 5 Requirement Levels: required, recommended, elective, limited use and not recommended.

• Required. An RFC is labeled required if it must be implemented by all Internet systems to achieve minimum conformance. For example, IP and ICMP are required protocols.
• Recommended. An RFC labeled recommended is not required for minimum conformance; it is recommended because of its usefulness. For example, FTP and TELNET are recommended protocols.
• Elective. An RFC labeled elective is not required and not recommended. However, a system can use it for its own benefit.
• Limited Use. An RFC labeled limited use should be used only in limited situations. Most of the experimental RFCs fall under this category.
• Not recommended. An RFC labeled not recommended is inappropriate for general use. Normally a historic (obsolete) RFC may fall under this category.

RFCs can be found at http://www.faqs.org/rfcs

Related Post

• Protocols and standards.
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Topology- definition and types of topology

A Topology of the network defines the manner in which the network devices are arranged and connected to each other in a network. It defines the shape of communication network. There are five common types of network Topologies.

1. Bus Topology
2. Ring Topology
3. Star Topology
4. Tree Topology
5. Mesh Topology

Bus Topology/ Linear Topology
In a bus topology a single main cable connects each node (computers) which allows single line of computers accessing it from end-to-end. each node is connected to two others except those in end. The network operating system keeps track of  a unique electronic address for each node in the network, and manages the flow of data based on this addressing scheme. This topology is often found in a client / server systems, where one of the machines on the network is designated as file server.

In linear bus topology, all computers are connected by a single length of cabling with a terminator at each end. the bus topology is the simplest and most widely used network design.
Bus networks are the most common LANs. they have no switches, and in their simplest form, no repeaters, but simply share a common linear communication medium. Each station requires a tap (hardware for attachment to the medium), which must be capable of delivering the signal to all stations in the bus.
 The data is sent in packets, and each station hears all the transmissions, picking up those addressed to it.

Advantages of Bus Topology

• Most bus networks have the advantage of being passive i.e all the active components are in the stations or nodes, and a failure affects only that one node.
• It does not require all the computers to be up and running in order for network to function.

Disadvantages of Bus Topology

• Because single cable is dedicated to all the computers the performance can suffer at time because of heavy traffic.
• There is a distance limitation in bus topology. After certain length of cable the  performance of the Bus network degrades.

Ring Topology/ Circular Topology
In ring topology the computers are arranged in a circle. Data travels around the ring in one direction, with each devise on the ring acting as a repeater. Ring Networks typically use a Token Passing Protocol.
The layout is similar to linear bus, except that the nodes are connected in a circle using cable segments. In this layout, each node is connected to only two others. Each node passes information along to the next, until it reaches at its intended destination.
The ring topology is usually found in  peer-to-peer (PCs connected in pairs) networks, in which each machine manages both information processing and distribution of data files.

In ring topology type LAN architecture a series of devices are connected to one another by unidirectional transmission links to form a single closed loop. Both token ring/ IEEE 8019.5 and FDDI networks implement a ring topology.

Advantages of ring topology

• Performance is good because each portion of cabling system is handling the data flow between two nodes (machines) only.
• They do not have distance limitations as in Bus topology (difference between Bus and ring topology).
• They can take advantages of fiber optic cables to speed up the performance, because only two machines are involved in packet exchange at a time.

Disadvantages of Ring Topology

• Since all the nodes or computers are involved in data transfer, the failure of single node can bring whole network to the halt.
• The ring control mechanism required to determine as to who should start up the ring, to determine that the packets are not corrupt, and to prevent the same packet to go around the ring because of network fault. Some Ring LANs need to deploy special computer to monitor this issue.

STAR Topology
In Star Topology, all the cables run from the computers to a central location, where they are connected by a hub. Hub is a device used to extend a network so that additional work stations can be attached.

In Star topology each node is connected to single centrally located server, using its own dedicated segment of cable. A star topology is a LAN architecture in which endpoints on the network are connected to a common central hub, or switch, by dedicated links. In this topology each node is connected to a centralised switch by a dedicated physical link. The switch provides a path between any two devices wishing to communicate, either physically in a circuit switch or logically in a packet switch.

Advantages Of star topology

• This topology has the advantage of minimum data traffic along the cables (node to server only)., thus providing optimum performance.
• The main advantage of star LAN are that the access to the network i.e decision on when a station can or cannot transmit, is under central control.

Disadvantages of Star Topology

• Because single central machine must coordinate all communications, this topology requires an extremely powerful server. Hence Star Topology is expensive.
• Speed is generally limited and central switch is an obvious potential source of catastrophic failure i.e if centralised server fails, whole topology fails.

Tree Topology
This is a network topology containing zero or more nodes/computers linked together in a hierarchical fashion. The topmost node is called the root. The root may have zero or more child nodes, connected by edges (links); the root is the parent root to its children. Each node can have in turn zero or more nodes of its own. Nodes sharing the same parents is called siblings. Every node in the tree has exactly one parent node (except root which has no parents), and all nodes in the tree are descendants of the root node. These relationships ensure that there is one and only one path from one node to any other node in the tree.
 A tree topology LAN architecture is identical to BUS topology network, except that branches with multiple nodes are possible in this case.
The advantages and disadvantages of Tree topology are same as that of Bus Topology.

Mesh Topology/ Graph Topology
In this topology, two or more nodes are connected together in an arbitrary fashion. Any two nodes in a Mesh or Graph may or may not be connected by a link. Not all the nodes need to be connected in a graph, but if the path can be traced between any two nodes, the graph is a connected one.
A Mesh Topology is a Mixture of BUS topology, STAR Topology, Ring and Tree Topology, with no restriction of connection among all the nodes in a network.

What is DNS- facts about DNS

Summary of DNS

• Domain name system (DNS) is a client-server application that identifies each host on the internet with a unique user-friendly name.
• DNS organizes the name space in a hierarchichal structure to decentralize the responsibilities involved in naming.
• DNS can be pictured as an inverted hierarchical tree structure with one root noed at the top and a maximum of 128 levels.
• Each node in the tree has a domain name.
• A domain is defined as any subtree of the domain name space.
• The name space information is distributed among DNS servers. Each server has jurisdiction over its zone.
• A root server's zone is the entire DNS tree.
• A primary server creates, maintains, and updates information about its zone.
• A secondary server gets itsinformation form the primary server.
• The domain name space is divided into three sections: generic domains, country domains, and inverse domain.
• There are seven generic domains, each specifying the organization type.
• Each country domain specifies a country.
• The inverse domain finds a domain name for the given IP address. This is called adress-to-name resolution.
• Name servers, computers that run the DNS server program, are organized in a hierarchy.
• The DNS Client, called a resolver, maps a name to an address or an address to a name.
• In recursive resolution, the client sends its request to a server that eventually returns a response.
• In iterative resolution, the client may send its request to multiple servers before getting an answer.
• Caching is a method whereby an answer to a query is stored in memory (for a limited time) for easy access to future requests.
• A fully qualified domain name (FQDN) is a domain name consisting of names begining with the host and going back through each level to the root node.
• A partially qualified domain name (PQDN) is a domain name that does not include all the levels between the host and the root node.
• There are two types of DNS messages: queries and responses.
• There are two types of DNS records: question records and resource records.
• DNS uses an offset pointer for duplicated domain name information in its messages.
• Dynamic DNS (DDNS) automatically updates the DNS master file.
• DNS uses the services of UDP for messages of less than 512 bytes; otherwise; TCP is used.

Above were some of the most important facts about the Domain name.

DNS Messages Format

DNS has two types of messages: query and response. Both types have the same format. The query message consists of a header and the question records; the response message consisits of a header, question records, answer records, authoritative records, and additional records.

Header
 Both query and response messages have the same header format with some fields set to zero for the query messages. the header is 12 byte and its format is as follows:

Identification	flags
Number of question records	Number of answer records(All os in query message
Number of authoritative records(All os in query message)	Number of additional records(All os in query message)

The header fields are as follows:

• Identification.  16 bit field used by the client to match the response with the query. The client uses ientification number each time it sends a query. the server duplicates this number in response.
• Flags. 16 bit field consisting of other subfields as shown below.

1. QR (query/response). If set (1) means message is a response , if 0 it means message type is query.
2. OpCode. 4-bit defines type of query or response (0-standard, 1-inverse, 2-server status required).
3. AA (authoritative answer). (1-bit and used inly in response message. Set (1)-means Authoritative server).
4. TC (truncated) . if set means value of 1, means messgae was more than 512 bytes and is truncated.
5. RD (recursion desired). A 1-bit field, when set means client desires reursive answer. It is repeated in both request and response.
6. RA (recursion available). 1-bit, and it is set only in response message to indiacate that recursion is available.
7. Reserved. A 3-bit subfield set to 000.
8. rCode. A 4-bit field which shows the status of error in the response. Of course, only an authoritative server can make such judgement.

Values of rCode:

1. 0 - No error
2. 1 - Format error.
3. 2 - Problem at name server.
4. 3 - Domain reference problem.
5. 4 - Quert type not supported.
6. 5 - Administratively prohibited.
7. 6-15 - Reserved

• Number of Question Records. This is a16-bit field consisting of number of queries in question section of message.
• Number of Answer Records. This is a 16-bit field containing the number of answer records in the answer section of response message. Its value is 0in the query message.
• Number of authoritative records. A sixteen bit field which tells the number of authoritative records in the authoritative section of the response message. Its value is zero in the query message.
• Number of additional records. This is a 16 bit fieldcontaining the number of additional recordsin the additional section of the response message.

Question Section
 This is a section consisting of one or more question records. It is present on both query and response messsages

Answer Section
This is section consisting of one or more resource records. It is present only in response messages. This section includes answer from the server to the client (resolver).

Authoritative Section
 This section is also contained only in response messages of DNS, and gives information about domain names regarding authoritative servers for the query.

Additional Information Section
 This section provides additional information to help the resolver and present only in response part of DNS message format.

This was discussion about the format of DNS message. We discussed about various sections of Domain Name System message format like header, question section, answer section, authoritative section, and Additional infor mation section of Domain Name System (DNS) messages.

Domain name-address resolution

Mapping a domain name to a physical IP address or an IP address to a domain name is called name-address resolution.

Resolver
DNS is designed as a client-server application. A host needs to map an address to a name or a name to an address calls a DNS client called resolver. The resolver accesses the closest DNS server with a mapping request. If the server has the information, it satisfies the resolver; otherwise, it either refers the resolver to other servers or asks other servers to provide the information.
After the resolver recieves the mapping, it interprets the response to see if it is a real resolution or an error, and finally delivers the result to the process that requested it.

Mapping Domain names to Addresses
Most of the time, the resolver gives a domain name to the server and asks for the corresponding address. In this case, the server checks the generic domains or the country domains to find the mapping.

If  the domain name is from the genric domains section, the resolver receives a domain name such as "chal.atc.fhda.edu.". The query is sent by the resolver to local DNS server for resolution. If the local server cannot resolve the query, it either refers the resolver to other servers or asks other servers directly.

If the domain name is from the country domains section, the resolver receives a domain name such as "ch.fhda.cu.ca.us.". The procedure is the same.

Mapping Addresses to Names
A client can send an IP address to a server to be mappedto a domain name. As mentioned before, this is called a PTR query. To answer queries of this kind, DNS uses the inverse domain. However, in the request, the IP address should be reversed and two labels, in-addr and arpa, should be appended to create a domain acceptable by inverse domain section. For example, If the resolver receives the Ip address 132.34.45.121, the resolver first inverts the address and then adds the two labels before sending. The domain name sent is "121.45.34.132.in-addr.arpa." , which is received by local DNS and resolved.

Recursive Resolution
The client (resolver) an ask for a recursive answer from a name server. This means that the resolver expects the server to supply the final answer. If the server is the authority for the domain name, it checks its database and responds. If the server is not the authority, it sends the request to yet another server. When the query is finally resolved, the response levels back until itfinally reaches the requesting client.

Iterative Resolution
 If the client does not ask for a recursiveanswer, the mapping can be done iteratively. If the server is an authority for the name, it sends the answer. If it is not, it returns(to the client) the IP address of the server that it thinks can resolve the query. The client is responsible for repeating the query to this second server. If the newly addressed server can resolve the problem, it answers the query with the IP address; otherwiese, it returns the IP address of a new server to the client. Now the client must repeat the query to the third server. thi sprocess is called iterative  because theclient repeats the same query to multiple servers.

Domain Name Caching (Cached Version of Page)
Each time a server recieves a query from a name that is not in its domain, it needs to search its database for a server IP address. Reduction of this search time would increase efficiency. DNS handles this with a mechanism called caching.  When a server asks for a mapping from another server and receives the response, it stores this information in its cache memory before sending it to the client. If the same or another client asks for the same mapping, it can check its cache memory and resolve the problem. However,to inform the client that response is coming from the cache memory and not from an authitative source, the server marks the response as unauthoritative.

 Caching speeds up resolution, but it can also be problematic. If a server cache a mapping for a long time, it may send an outdated mapping to the client. To counter this, two techniques are used. First, the authoritative server always adds a piece of information to the mapping called time-to-live (TTL). It defines the time in seconds that the reciever server can cache the information. After that time, the mapping is invalid and any query must be sent again to the authoritative server. Second, DNS requires that each server keeps a TTL counter for each mapping it caches. The cache memory must be searched periodically and those mappings with an expired TTL must be purged.

That was the discussion on Name-address resolution in DNS, Iterative and recursive resolution of domain addresses. We also learned about Caching and Domain name mappings, & TTL. Hope you have enjoyed reading the article.

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DNS In the Internet Domain

DNS is a protocol that can be used in different platforms. in the Internet, the Domain name space is divided into three different sections: generic domains, country domains, and inverse domain.

Generic Domain
The generic domain define registered hosts according to their generic behavior. Each node in the tree defines a domain, which is an index to the domain name space database.

First level in generic domain section allows seven possible three-character labels (or extensions). These labels describes the organization types as listed:

Label	Description
.com	Commercial organizations
.edu	Educational Institutions
.gov	Government Institutions
.int	International organizations
.Mil	Military groups
.net	Network support centers
.org	Nonprofit organizations

recently a few more first-level domain names heve been approved:

Label	Description
.aero	Airlines and aerospace companies
.biz	Business or firms (similar to ".com")
.coop	Cooperative Business Organization
.info	Information service providers
.museum	Museums and other non profit organizations
.name	Personal names (individuals)
.org	Nonprofit organizations
.pro	Professional Individual organizations

Country Domain:
The country domain uses the same format as generic domains but uses two-character country abbreviations (eg., .us fro United States, .in for India), in place of three-character organizational abbreviations at the foiat level. Second-Level labels can be organizational, or they can be more specific, national designations. The United States, For example, uses state abbreviations as a subdivision of us (e.g., .ca.us.).

Inverse Domain
The inverse domain is used to map an address to a name. This may happen,, for example, when as server has received a request from client to do a task. Whereas the server has a file that contains a list of authorized clients, the serve lists only the Ip address of the client (extracted from received IP packet). To determine if the client is on the authorized list, it can ask its resolver to send a query to the DNS server and ask for a maping of address to name.
This type of query is called an inverse or pointer (PTR) query. To handle a pointer query, the inverse domain is added to the domain name space with the first-level node called arpa ( for historical reasons). the second level is also one single node named in-addr (for inverse address). The rest of the domain defines IP addresses.

The servers that handle the inverse domains are also hierarchichal. this means the netid part of the address should be at higher level than the subnetid part, and the subnetid part higher than the hostid part. In this way, a server serving the whole site is at a higher level than the servers serving each subnet. this configuration makes the domain look inverted when compared to a gneric or country domain.

Example of Inverse Domain
To follow the convention of reading the domain names from the bottom to the top, an IP address such as 132.34.45.121 9a class B address with netid 132.34) is read as 121.45.34.132.inaddr.arpa.

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