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NETWORK BASICS

Network A system of interconnected computers and computerized peripherals such as printers is called computer network. This interconnection among computers facilitates information sharing among them. Computers may connect to each other by either wired or wireless media. A computer network consists of a collection of computers, printers and other equipment that is connected together so that they can communicate with each other.  


Network application
A Network application is any application running on one host and provides a communication to another application running on a different host, the application may use an existing application layer protocols such as: HTTP(e.g. the Browser and web server), SMTP(e.g. the email-client). And may be the application does not use any existing protocols and depends on the socket programming to communicate to another application. So the web application is a type of the network applications. 
There are lots of advantages from build up a network, but the th…

NETWORK DEVICES

Image result for NETWORK DEVICES"Computer networking devices are units that mediate data in a computer network and are also called network equipment. Units which are the last receiver or generate data are called hosts or data terminal equipment. Home networks generally have only one WAP to connect all the computers in a home. Most are wireless routers, meaning converged devices that include a WAP, router, and often an Ethernet switch in the same device. Many also converge a broadband modem. 

  • HU
  • Switch 
  • Repeater 
  • Modem 
  • NIC(network Interface card) 
  • Transceiver 
  • Bridge 
  • Router 
  • Firewall 

When referring to a network, a hub is the most basic networking device that connects multiple computers or other network devices together. Unlike a network switch or router, a network hub has no routing tables or intelligence on where to send information and broadcasts all network data across each connection. Most hubs can detect basic network errors such as collisions, but having all information broadcast to multiple ports can be a security risk and cause bottlenecks. In the past network hubs were popular because they were cheaper than a switch and router. Today, switches do not cost much more than a hub, and are a much better solution for any network. In general, a hub refers to a hardware device that enables multiple devices or connections to be connected to a computer. Another example besides the one given above is a USB hub, which allows dozens of USB devices to be connected to one computer, even though that computer may only have a few USB connections.  

  • Active HUB 
  • Passive HUB 
ACTIVE HUB:
Active hubs are a little smarter than passive hubs. You might also come across the term "concentrators," which are basically active hubs that concentrate and strengthen a signal as it enters and exits the hub. 

These hubs are nothing more than point contacts for the wires that make up the physical network. An example of this is a punch-down block that is a simple plastic, unpowered box used to plug network cables into. 

A network switch (also called switching hub, bridging hub, officially MAC Bridge) is a computer networking device that connects devices together on a computer network, by using packet switching to receive, process and forward data to the destination device. A network switch is a multi-port network bridge that uses hardware addresses to process and forward data at the data link layer (layer 2) of the OSI model. Switches can also process data at the network layer (layer 3) by additionally incorporating routing functionality that most commonly uses IP addresses to perform packet forwarding; such switches are commonly known as layer-3 switches or multi-layer switches. On an Ethernet local area network (LAN), a switch determines from the physical device (Media Access Control or MAC) address in each incoming message frame which output port to forward it to and out of. In a wide area packet-switched network such as the Internet, a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination. 


A repeater is a device that amplifies the signal from a wireless network and operates in a combination of bridging and access point modes. A repeater takes a signal from a wireless access point or router and broadcasts it to wireless clients. We're particularly fond of repeaters because they are simple to install and to move around as you need to. A repeater does not connect to an Ethernet port, except for when you are configuring it, so these devices are entirely portable. A repeater can help you get your wireless signal to a remote bedroom, the basement, or your patio, and may allow a small network to use a single access point. Repeaters take the same settings as the device they are repeating and they don't add much to the complexity of a wireless network. Strangely, most wireless books and articles don't spend a lot of time describing repeaters, probably because they are only recently appearing in consumer wireless devices. Normally you don't buy a repeater perse; you buy an access point that can be placed into repeater mode.  

The first generation of access points did not offer a repeater mode, but most of the current crop of access points do have this feature. A repeater is also called a range extender and, depending upon the protocol and placement, can amplify the signal 50 percent or more. The theoretical specification for the range of the DWL-G800AP repeater is from 328 feet (100 meters) indoors up to 1,312 feet (400 meters) outdoors. Repeaters remove the unwanted noise in an incoming signal. Unlike an analog signal, the original digital signal, even if weak or distorted, can be clearly perceived and restored. With analog transmission, signals are strengthened with amplifiers which unfortunately also amplify noise as well as information. Because digital signals depend on the presence or absence of voltage, they tend to dissipate more quickly than analog signals and need more frequent repeating. Whereas analog signal amplifiers are spaced at 18,000 meter intervals, digital signal repeaters are typically placed at 2,000 to 6,000 meter intervals. In a wireless communications system, a repeater consists of a radio receiver, an amplifier, a transmitter, an isolator, and two antennas. The transmitter produces a signal on a frequency that differs from the received signal. This so-called frequency offset is necessary to prevent the strong transmitted signal from disabling the receiver. The isolator provides additional protection in this respect. A repeater, when strategically located on top of a high building or a mountain, can greatly enhance the performance of a wireless network by allowing communications over distances much greater than would be possible without it. 


A modem modulates outgoing digital signals from a computer or other digital device to analog signals for a conventional copper twisted pair telephone line and demodulates the incoming analog signal and converts it to a digital signal for the digital device. In recent years, the 2400 bits per second modem that could carry e-mail has become obsolete. 14.4 Kbps and 28.8 Kbps modems were temporary landing places on the way to the much higher bandwidth devices and carriers of tomorrow. From early 1998, most new personal computers came with 56 Kbps modems. By comparison, using a digital Integrated Services Digital Network adapter instead of a conventional modem, the same telephone wire can now carry up to 128 Kbps. With Digital Subscriber Line (DSL) systems, now being deployed in a number of communities, bandwidth on twisted-pair can be in the megabit range.  


A network interface card provides the computer with a dedicated, full-time connection to a network. Personal computers and workstations on a local area network (LAN) typically contain 
A network interface card specifically designed for the LAN transmission technology. Due to the popularity and low cost of the Ethernet standard, most new computers have a network interface build directly into the motherboard. Most new computers have either Ethernet capabilities integrated into the motherboard chip-set, or use an inexpensive dedicated Ethernet chip connected through the PCI or PCI Express bus. A separate NIC is generally no longer needed. If the card or controller is not integrated into the motherboard, it may be an integrated component in a router, printer interface or USB device. Typically, there is an LED next to the connector informing the user if the network is active or whether or not data is being transferred on it. Depending on the card or motherboard, transfer rates may be 10, 100, or 1000 Megabits per second. 


A transceiver is a combination transmitter/receiver in a single package. The term applies to wireless communications devices such as cellular telephones, cordless telephone sets, handheld two-way radios, and mobile two-way radios. Occasionally the term is used in reference to transmitter/receiver devices in cable or optical fiber systems. In a radio transceiver, the receiver is silenced while transmitting. An electronic switch allows the transmitter and receiver to be connected to the same antenna, and prevents the transmitter output from damaging the receiver. With a transceiver of this kind, it is impossible to receive signals while transmitting. This mode is called half duplex. Transmission and reception often, but not always, are done on the same frequency. Some transceivers are designed to allow reception of signals during transmission periods.  

This mode is known as full duplex, and requires that the transmitter and receiver operate on substantially different frequencies so the transmitted signal does not interfere with reception. Cellular and cordless telephone sets use this mode. Satellite communications networks often employ full-duplex transceivers at the surface-based subscriber points. The transmitted signal (transceiver-to-satellite) is called the up-link, and the received signal (satellite-to-transceiver) is called the down-link.  

A bridge is a type of computer network device that provides interconnection with other bridge networks that use the same protocol. Bridge devices work at the data link layer of the Open System Interconnect (OSI) model, connecting two different networks together and providing communication between them. Bridges are similar to repeaters and hubs in that they broadcast data to every node. However, bridges maintain the media access control (MAC) address table as soon as they discover new segments, so subsequent transmissions are sent to only to the desired recipient. Bridges are also known as Layer 2 switches.  

A network bridge device is primarily used in local area networks because they can potentially flood and clog a large network thanks to their ability to broadcast data to all the nodes if they don’t know the destination node's MAC address.  

A bridge uses a database to ascertain where to pass, transmit or discard the data frame. 

1. If the frame received by the bridge is meant for a segment that resides on the same host network, it will pass the frame to that node and the receiving bridge will then discard it.  

2. If the bridge receives a frame whose node MAC address is of the connected network, it will forward the frame toward it.  


A router is hardware device designed to receive, analyze and move incoming packets to another network. It may also be used to convert the packets to another network interface, drop them, and perform other actions relating to a network. The picture shows the Linksys BEFSR11 wireless router and is what many home routers resemble. A router has a lot more capabilities than other network devices, such as a hub or a switch that are only able to perform basic network functions. For example, a hub is often used to transfer data between computers or network devices, but does not analyze or do anything with the data it is transferring. By contrast, routers can analyze the data being sent over a network, change how it is packaged, and send it to another network or over a different network.  
For example, routers are commonly used in home networks to share a single Internet connection between multiple computers. Technically, a router is a Layer 3 gateway device, meaning that it connects two or more networks and that the router operates at the network layer of the OSI model. Home networks typically use a wireless or wired Internet Protocol (IP) router, IP being the most common OSI network layer protocol. An IP router such as a DSL or cable modem broadband router joins the home's local area network (LAN) to the wide-area network (WAN) of the Internet. A router may create or maintain a table of the available routes and their conditions and use this information along with distance and cost algorithms to determine the best route for a given packet. Typically, a packet may travel through a number of network points with routers before arriving at its destination. Routing is a function associated with the Network layer (layer 3) in the standard model of network programming, the Open Systems Interconnection (OSI) model. A layer-3 switch is a switch that can perform routing functions. An edge router is a router that interfaces with an asynchronous transfer mode (ATM) network. Ab router is a network bridge combined with a router. For home and business computer users who have high-speed Internet connections such as cable, satellite, or DSL, a router can act as a hardware firewall. This is true even if the home or business has only one computer. Many engineers believe that the use of a router provides better protection against hacking than a software firewall, because no computer Internet Protocol address are directly exposed to the Internet. This makes port scans (a technique for exploring weaknesses) essentially impossible. In addition, a router does not consume computer resources as a software firewall does. Commercially manufactured routers are easy to install, reasonably priced, and available for hardwired or wireless networks. 


A firewall is a network security system, either hardware- or software-based, that controls incoming and outgoing network traffic based on a set of rules. It is acting as a barrier between a trusted network and other untrusted networks such as the Internet or less-trusted networks such as a retail merchant's network outside of a cardholder data environment a firewall  Controls access to the resources of a network through a positive control model. This means that the only traffic allowed onto the network defined in the firewall policy is; all other traffic is denied. Computer security borrowed the term firewall from firefighting and fire prevention, where a firewall is a barrier established to prevent the spread of fire. 

When organizations began moving from mainframe computers and dumb clients to the client-server model, the ability to control access to the server became a priority. Before firewalls emerged in the late 1980s, the only real form of network security was performed by access control lists (ACLs) residing on routers. ACLs determined which IP addresses were granted or denied access to the network. Firewalls are frequently used to prevent unauthorized Internet users from accessing private networks connected to the Internet, especially intranets. All messages entering or leaving the intranet pass through the firewall, which examines each message and blocks those that do not meet the specified security criteria. 


The earliest firewalls functioned as packet filters, inspecting the packets that are transferred between computers on the Internet. When a packet passes through a packet-filter firewall, its source and destination address, protocol, and destination port number are checked against the firewall's rule set. Any packets that aren't specifically allowed onto the network are dropped (i.e., not forwarded to their destination). For example, if a firewall is configured with a rule to block Telnet access, then the firewall will drop packets destined for TCP port number 23, the port where a Telnet server application would be listening. Packet-filter firewalls work mainly on the first three layers of the OSI reference model (physical, data-link and network), although the transport layer is used to obtain the source and destination port numbers. While generally fast and efficient, they have no ability to tell whether a packet is part of an existing stream of traffic. Because they treat each packet in isolation, this makes them vulnerable to spoofing attacks and also limits their ability to make more complex decisions based on what stage communications between hosts are at. 


In order to recognize a packet's connection state, a firewall needs to record all connections passing through it to ensure it has enough information to assess whether a packet is the start of a new connection, a part of an existing connection, or not part of any connection. This is what's called "stateful packet inspection." Stateful inspection was first introduced in 1994 by Check Point Software in its Fire Wall-1 software firewall, and by the late 1990s, it was a common firewall product feature. This additional information can be used to grant or reject access based on the packet's history in the state table, and to speed up packet processing; that way, packets that are part of an existing connection based on the firewall's state table can be allowed through without further analysis. If a packet does not match an existing connection, it's evaluated according to the rule set for new connections. 


As attacks against Web servers became more common, so too did the need for a firewall that could protect servers and the applications running on them, not merely the network resources behind them. Application-layer firewall technology first emerged in 1999, enabling firewalls to inspect and filter packets on any OSI layer up to the application layer. The key benefit of application-layer filtering is the ability to block specific content, such as known malware or certain websites, and recognize when certain applications and protocols such as HTTP, FTP and DNS are being misused. Firewall technology is now incorporated into a variety of devices; many routers that pass data between networks contain firewall components and most home computer operating systems include software-based firewalls. Many hardware-based firewalls also provide additional functionality like basic routing to the internal network they protect. 


Firewall proxy servers also operate at the firewall's application layer, acting as an intermediary for requests from one network to another for a specific network application. A proxy firewall prevents direct connections between either sides of the firewall; both sides are forced to conduct the session through the proxy, which can block or allow traffic based on its rule set. A proxy service must be run for each type of Internet application the firewall will support, such as an HTTP proxy for Web services. 


The Dynamic Host Configuration Protocol (DHCP) is a network protocol used to assign IP addresses and provide configuration information to devices such as servers, desktops, or mobile devices, so they can communicate on a network using the Internet Protocol (IP). ISC DHCP is a collection of software that implements all aspects of the DHCP (Dynamic Host Configuration Protocol) suite. It includes: 
  • A DHCP server, which receives clients’ requests and replies to them. 
  • A DHCP client, which can be bundled with the operating system of a client computer or other IP capable device and which sends configuration requests to the server. Most devices and operating systems already have DHCP clients included. 
  •  A DHCP relay agent, which passes DHCP requests from one LAN to another so that there need not be a DHCP server on every LAN. 
The DHCP server, client and relay agent are provided both as reference implementations of the protocol and as working, fully-featured sample implementations. Both the client and the server provide functionality that, while not strictly required by the protocol, is very useful in practice. The DHCP server also makes allowances for non-compliant clients that need to be supported. The ISC DHCP server will answer requests from any client that complies with the protocol standards, and the ISC DHCP client can interact with any server that complies with those standards. The components of ISC DHCP need not all be used together. Every device on a TCP/IP-based network must have a unique uni-cast IP address to access the network and its resources. Without DHCP, IP addresses for new computers or computers that are moved from one subnet to another must be configured manually; IP addresses for computers that are removed from the network must be manually reclaimed.  With DHCP, this entire process is automated and managed centrally. The DHCP server maintains a pool of IP addresses and leases an address to any DHCP-enabled client when it starts up on the network. Because the IP addresses are dynamic (leased) rather than static (permanently assigned), addresses no longer in use are automatically returned to the pool for reallocation. The network administrator establishes DHCP servers that maintain TCP/IP configuration information and provide address configuration to DHCP-enabled clients in the form of a lease offer. The DHCP server stores the configuration information in a database that includes: 
  • Valid TCP/IP configuration parameters for all clients on the network. 
  • Valid IP addresses, maintained in a pool for assignment to clients, as well as excluded addresses. 
  • Reserved IP addresses associated with particular DHCP clients. This allows consistent assignment of a single IP address to a single DHCP client. 
  • The lease duration, or the length of time for which the IP address can be used before a lease renewal is required. 
  • A DHCP-enabled client, upon accepting a lease offer, receives: 
  • A valid IP address for the subnet to which it is connecting. 
  • Requested DHCP options, which are additional parameters that a DHCP server is configured to assign to clients. Some examples of DHCP options are Router (default gateway), DNS Servers, and DNS Domain Name. For a full list of DHCP options, see DHCP Tools and Options.  
DHCP is an extension of an earlier network IP management protocol, Bootstrap Protocol (BOOTP). DHCP is more advanced, and DHCP servers can handle BOOTP client requests if any BOOTP clients remain on a network segment. DHCP is not a routable protocol; it is limited to a specific local area network (LAN). If network administrators want a given DHCP server to provide addressing to multiple subnets on a given network, they must configure DHCP relay services on the routers DHCP requests have to cross. DHCP is not a secure protocol, as no mechanism is built in to allow clients and servers to authenticate each other. Both are vulnerable to deception (e.g., one computer can pretend to be another) and to attack (rogue clients can exhaust a server’s address pool). 


If you've ever used the Internet, it's a good bet that you've used the Domain Name System, or DNS, even without realizing it. DNS is a protocol within the set of standards for how computers exchange data on the Internet and on many private networks, known as the TCP/IP protocol suite. Its basic job is to turn a user-friendly domain name like "howstuffworks.com" into an Internet Protocol (IP) address like 70.42.251.42 that computers use to identify each other on the network. It's like your computer's GPS for the Internet. Computers and other network devices on the Internet use an IP address to route your request to the site you're trying to reach. This is similar to dialing a phone number to connect to the person you're trying to call. Thanks to DNS, though, you don't have to keep your own address book of IP addresses. Instead, you just connect through a domain name server, also called a DNS server or name server, which manages a massive database that maps domain names to IP addresses. Whether you're accessing a Web site or sending e-mail, your computer uses a DNS server to look up the domain name you're trying to access. The proper term for this process is DNS name resolution, and you would say that the DNS server resolves the domain name to the IP address. For example, when you enter "http://www.yahoo.com" in your browser, part of the network connection includes resolving the domain name "howstuffworks.com" into an IP address, like 70.42.251.42, for How Stuff Works' Web servers. You can always bypass a DNS lookup by entering 70.42.251.42 directly in your browser (give it a try). However, you're probably more likely to remember "howstuffworks.com" when you want to return later. In addition, a Web site's IP address can change over time, and some sites associate multiple IP addresses with a single domain name. Without DNS servers, the Internet would shut down very quickly. But how does your computer know what DNS server to use? Typically, when you connect to your home network, Internet service provider (ISP) or Wi-Fi network, the modem or router that assigns your computer's network address also sends some important network configuration information to your computer or mobile device. That configuration includes one or more DNS servers that the device should use when translating DNS names to IP address. 


The Domain name system comprises of Domain Names, Domain Name Space, and Name Server that have been described below: 

Domain Name is a symbolic string associated with an IP address. There are several domain names available; some of them are generic such as com, edu, gov, net etc, while some country level domain names such as au, in, za, us etc. 


Com
Commercial business
Edu
Education
Gov
U.S. government agency
Int
International entity
Mil
U.S. military
Net
Networking organization
Org
Non-profit organization

The following table shows the Country top-level domain names: 

au
Australia
in
India
cl
Chile
fr
France
us
United States
za
South Africa
uk
United Kingdom
jp
Japan
es
Spain
de
Germany
ca
Canada
ee
Estonia
hk
Hong Kong


The domain name space refers a hierarchy in the internet naming structure. This hierarchy has multiple levels (from 0 to 127), with a root at the top. The following diagram shows the domain name space hierarchy: 

In the above diagram each sub tree represents a domain. Each domain can be partitioned into sub domains and these can be further partitioned and so on. 


Name server contains the DNS database. This database comprises of various names and their corresponding IP addresses. Since it is not possible for a single server to maintain entire DNS database, therefore, the information is distributed among many DNS servers. 
  • Hierarchy of server is same as hierarchy of names. 
  • The entire name space is divided into the zones 

Zone is collection of nodes (sub domains) under the main domain. The server maintains a database called zone file for every zone. If the domain is not further divided into sub domains then domain and zone refers to the same thing. 

The information about the nodes in the sub domain is stored in the servers at the lower levels however; the original server keeps reference to these lower levels of servers. 
Types of Name Servers 


I. Root Server 
II. Primary Server 
III. Secondary Server 


Root Server is the top level server which consists of the entire DNS tree. It does not contain the information about domains but delegates the authority to the other server 


Primary Server stores a file about its zone. It has authority to create, maintain, and update the zone file. 


Secondary Server transfers complete information about a zone from another server which may be primary or secondary server. The secondary server does not have authority to create or update a zone file. 
How DNS works 
When you visit a domain such as google.com your computer follows a series of steps to turn the human-readable web address into a machine-readable IP address. This happens every time you use a domain name, whether you are viewing websites, sending email or listening to Internet radio stations. 


The process begins when you ask your computer to resolve a host-name, such as visiting http://www.google.com. The first place your computer looks is its local DNS cache, which stores information that your computer has recently retrieved. 
If your computer doesn’t already know the answer, it needs to perform a DNS query to find out. 


If the information is not stored locally, your computer queries (contacts) your ISP’s recursive DNS servers. These specialized computers perform the legwork of a DNS query on your behalf. Recursive servers have their own caches, so the process usually ends here and the information is returned to the user. 


If the recursive servers don’t have the answer, they query the root name servers. A name server is a computer that answers questions about domain names, such as IP addresses. The thirteen root name servers act as a kind of telephone switchboard for DNS. They don’t know the answer, but they can direct our query to someone that knows where to find it. 


The root name servers will look at the first part of our request, reading from right to left — www.google.com and direct our query to the Top-Level Domain (TLD) name servers for .com. Each TLD, such as .com, .org, and .us, have their own set of name servers, which act like a receptionist for each TLD. These servers don’t have the information we need, but they can refer us directly to the servers that do have the information. 


The TLD name servers review the next part of our request www.google.com and direct our query to the name servers responsible for this specific domain. These authoritative name servers are responsible for knowing all the information about a specific domain, which are stored in DNS records. There are many types of records, which each contain a different kind of information. In this example, we want to know the IP address for www.google.com so we ask the authoritative name server for the Address Record (A). 


The recursive server retrieves the A record for www.google.com,from the authoritative name servers and stores the record in its local cache. If anyone else requests the host record for google.com the recursive servers will already have the answer and will not need to go through the lookup process again. All records have a time-to-live value, which is like an expiration date. After a while, the recursive server will need to ask for a new copy of the record to make sure the information doesn’t become out-of-date. 


Armed with the answer, recursive server returns the A record back to your computer. Your computer stores the record in its cache, reads the IP address from the record, and then passes this information to your browser. The browser then opens a connection to the web-server and receives the website. 
This entire process, from start to finish, takes only milliseconds to complete. 
 


A. Hub 
B. Switch  
C. Repeater  
D. All of the above  


A. Hub  
B. Wireless access point  
C. Switch  
D. Router 


A) Station address 
B) IP address 
C) Port address 
D) Checksum  


A. Router  
B. Switch  
C. Hub  
D. Bridge        


A. The DNS server 
B. The DHCP server  
C. The Proxy server  
D. The Firewall               


A. Hub  
B. Switch  
C. Router  
D. Modem  


A. Hub  
B. Switch  
C. Router  
D. Repeater 

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