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Internet and Technologies

The Internet is a global system of interconnected computer networks that use the Internet protocol suite (TCP/IP) to link several billion devices worldwide. It is a network of networks that consists of millions of private, public, academic, business, and government networks of local to global scope, linked by a broad array of electronic, wireless, and optical networking technologies. The Internet carries an extensive range of information resources and services, such as the inter-linked hypertext documents and applications of the World Wide Web (WWW), electronic mail, telephony, and peer-to-peer networks for file sharing.

The origins of the Internet date back to research commissioned by the United States government in the 1960s to build robust, fault-tolerant communication via computer networks. The primary precursor network, the ARPANET, initially served as a backbone for interconnection of regional academic and military networks in the 1980s. The funding of a new U.S. backbone by the National Science Foundation in the 1980s, as well as private funding for other commercial backbones, led to worldwide participation in the development of new networking technologies, and the merger of many networks. The linking of commercial enterprises by the early 1990s marks the beginning of the transition to the modern Internet, and generated a sustained exponential growth as generations of institutional, personal, and mobile computers were connected to the network.

Although the Internet has been widely used by academia since the 1980s, the commercialization incorporated its services and technologies into virtually every aspect of modern human life. As of 2014, 38 percent of the world's human population has used the services of the Internet within the past year—over 100 times more people than were using it in 1995. Internet use grew rapidly in the West from the mid-1990s to early 2000s and from the late 1990s to present in the developing world.

Most traditional communications media, including telephony and television, are being reshaped or redefined by the Internet, giving birth to new services such as Internet telephony and Internet television. Newspaper, book, and other print publishing are adapting to website technology, or are reshaped into blogging and web feeds. The entertainment industry, including music, film, and gaming, was initially the fastest growing online segment. The Internet has enabled and accelerated new forms of human interactions through instant messaging, Internet forums, and social networking. Online shopping has grown exponentially both for major retailers and small artisans and traders. Business-to-business and financial services on the Internet affect supply chains across entire industries.

The Internet has no centralized governance in either technological implementation or policies for access and usage; each constituent network sets its own policies. Only the overreaching definitions of the two principal name spaces in the Internet, the Internet Protocol address space and the Domain Name System (DNS), are directed by a maintainer organization, the Internet Corporation for Assigned Names and Numbers (ICANN). The technical underpinning and standardization of the core protocols is an activity of the Internet Engineering Task Force (IETF), a non-profit organization of loosely affiliated international participants that anyone may associate with by contributing technical expertise.


INTRANET

Intranet is defined as private network of computers within an organization with its own server and firewall. Moreover we can define Intranet as:

  • Intranet is system in which multiple PCs are networked to be connected to each other. PCs in intranet are not available to the world outside of the intranet.
  • Usually each company or organization has their own Intranet network and members/employees of that company can access the computers in their intranet.
  • Every computer in internet is identified by a unique IP address.
  • Each computer in Intranet is also identified by a IP Address, which is unique among the computers in that Intranet.


EXTRANET

Extranet refers to network within an organization, using internet to connect to the outsiders in controlled manner. It helps to connect businesses with their customers and suppliers and therefore allows working in a collaborative manner.

Extranet is implemented as a Virtual Private Networks (VPN) because it uses internet to connect to corporate organization and there is always a threat to information security. VPN offers a secure network in public infrastructure (Internet).


Gateway :

It is a particular point in a network that acts as an entrance to another network. For example, the 'payment gateway', if you've done any internet payment transaction you would have seen that from the merchant's site you are taken to a payment gateway, from where you are then taken to your bank's net banking site.


Firewall :

It is a program in charge of rejecting unwanted incoming connection requests. A server with a firewall will keep other computers form connecting to the server. It is like a boundary wall to keep intruders at bay.


DNS :

Domain name server - is a program which deals with computer addresses. You can look at it as a telephone directory, but for the Internet; it translates user friendly computer hostnames into IP addresses.


Domain Name

Domain names are used to identify one or more IP addresses. For example, the domain name microsoft.com represents about a dozen IP addresses. Domain names are used in URLs to identify particular Web pages. For example, in the URL http://www.prepjunkie.com/web/index.html, the domain name is prepjunkie.com.

Every domain name has a suffix that indicates which top level domain (TLD) it belongs to. There are only a limited number of such domains. For example:


in - India
gov - Government agencies
edu - Educational institutions
org - Organizations (nonprofit)
mil - Military
com - commercial business
net - Network organizations


Flash :

It is an embedded animation software/program that displays small animations on web pages; like the how many ducks can you shoot!?


URL :

Uniform Resource Locater - is nothing but a web page's address, or in other words URL specifies the address of every file on the internet.
URL has three parts
(i) the protocol to be used to access the file = {http://}
(ii) the IP address = {www.gmail}
(iii) the domain name = {.com}


Browser :

It is the software or program that allows a computer to view web pages. Like Mozilla Firefox, Google Chrome, Internet Explorer, Safari, Opera.


Web Crawler :

It is a program that visits web sites and reads their pages and other information in order to create entries for a search engine index.


Search Engines :

They are special program running on a website which offers user results of simultaneous searches of other websites for the required information. For example - Google


Traffic Congestion :

It is a phenomenon where too many users are connecting to the internet at the same time or the internet service provider's cables are not able to support the user's maximum speed; it really is like the traffic jam!


Digital Signature :

Also known as electronic signature is like an encryption technique for documents, which guarantees authenticity of the document and helps combat digital forgeries and frauds.

Encryption/ Decryption :

Encryption is a technique, derived from military use, to mask/disguise any information, which cannot be read by anyone who does not have the password/key. In other words, encryption is 'encoding' any message/information so that only authorizes people can read it. Encryption is done with a private key. Thus, decryption is the 'decoding' of the encoded message, with the help of a public key.


Cloud Computing :

It is a service provided, wherein one can obtain network based storage space with such service providers. User needs to open an account with the service provider, much like creating an e-mail id. E-mails are also cloud computing but on a smaller scale.

Cloud computing is the newest internet fascination which is attracting large user base from all around the world; virtual storage space is not only easier but also has many benefits:
(i) accessing documents/data from anywhere as long as you have internet connection,
(ii) no need to carry storage devices, like external hard disks,
(iii) online backups help when data are lost on existing systems,
(iv) for small businesses cloud storage is very helpful as it saves their costs of purchasing and maintaining storage devices and hardwares,
(v) you can buy space as per requirement, and cost of storage is lesser than other options,

Types of clouds:
(i) Public Cloud: it can be accessed by any subscriber with an internet connection and access to the cloud space.
(ii) Private Cloud: is where only an authorized group of individuals or organizations have limited access.
(iii) Community Cloud: like the name suggests is a ‘communal’ thing, where the service is shared among two or more organizations or companies, that have similar cloud requirements.
(iv) Hybrid Cloud: where a mixture of user interests are all catered to, i.e. a combination of public, private and community.

Types of cloud service providers:
(i)SaaS : Software as a Service provider gives access to users for resources as well as apps. Thus, the user does not need to store software or apps on their computer/smartphones/tablets. So, you can do your work using the software provided by the cloud service providers. User has least control over the cloud.
(ii)PaaS : Platform as a Service provider is a level above Saas; it gives users access and utilities(tools) to develop applications(apps) as per their requirement, save such apps and then use such apps. User has more control over the cloud.
(iii) IaaS : Infrastructure as a Service provider provides infrastructure; computer infrastructure is the hardware and software.

Important Application Layer's terms and definitions

Application Layer is responsible or communication between process over systems.

Client-server architecture
Client-server architecture
Server :
* always-on host     * permanent IP address     * server farms for scaling.
Clients :
* communicate with server     * may be intermittently connected     * may have dynamic IP addresses     * do not communicate directly with each other

Pure P2P architecture
* no always-on server
* arbitrary end systems directly communicate
* peers are intermittently connected and change IP addresses
* example: Gnutella

Different protocols supported by Application Layer :

HTTP
HTTP is “stateless” protocol server maintains no information about past client requests
Procedure :
* The client initiates TCP connection (creates socket) to server, port 80.
* The server accepts TCP connection from client.
* HTTP messages exchanged between browser (HTTP client) and Web server (HTTP server).
* TCP connection closed.

HTTP connections

Non Persistent HTTP
    At most one object is sent over a TCP connection.
Persistent HTTP
    Multiple objects can be sent over single TCP connection between client and server.
Persistent HTTP and Non Persistent HTTP

FTP : File Transfer Protocol
* FTP is used to transfer file to/from remote host.
* It has client/server model
    - client: side that initiates transfer (either to/from remote)
    - server: remote host

FTP: separate control, data connections

Procedure :
* FTP client contacts FTP server at port 21, specifying TCP as transport protocol.
* Client obtains authorization over control connection.
* Client browses remote directory by sending commands over control connection.
* When server receives a command for a file transfer, the server opens a TCP data connection to client.
* After transferring one file, server closes connection.
* Server opens a second TCP data connection to transfer another file.
* Control connection: “out of band”
* FTP server maintains “state”: current directory, earlier authentication.

Electronic Mail
Three major components:
* user agents
* mail servers
* simple mail transfer protocol: SMTP
User Agent
* It is also known as “mail reader”
* composing, editing, reading mail messages
* e.g. Outlook, elm, Netscape Messenger
* Outgoing, incoming messages stored on server
User Agent
* mailbox contains incoming messages for user
* message queue of outgoing (to be sent) mail messages
* SMTP protocol between mail servers to send email messages

Electronic Mail: SMTP
* SMTP uses TCP to reliably transfer email message from client to server, port 25
* Three phases of transfer (a) handshaking (greeting) (b) transfer of messages (c) closure
* SMTP uses persistent connections & requires message (header & body) to be in 7-bit ASCII
SMTP simple mail transfer protocol

Mail access protocols :
* SMTP: delivery/storage to receiver’s server
* Mail access protocol: retrieval from server
(a) POP: Post Office Protocol - authorization (agent <--*gt;server) and download
(b) IMAP: Internet Mail Access Protocol - more features & manipulation of stored message on server.
(c) HTTP: Hotmail , Yahoo! Mail, etc.

POP3 protocol :
* It is used to download mails that can be viewed on local machine using software such as Outlook, Thunderbird etc.
* When acknowledgement of message returns after downloading message, the message is deleted.

IMAP :
* Keep all messages in one place: the server
* Allows user to organize messages in folders
* IMAP keeps user state across sessions.

DNS: Domain Name System
DNS is used for Hostname to IP address translation, Host aliasing - Canonical and alias names, Mail server aliasing, Load distribution - replicated Web servers: set of IP addresses for one canonical name

Distributed, Hierarchical Database
DNS: Domain Name System
Procedure :
* Client queries a root server to find com DNS server.
* Client queries com DNS server to get prepjunkie.com DNS erver.
* Client queries amazon.com DNS server to get IP address for www.prepjunkie.com.
DNS: Root name servers
Root name servers are contacted by local name server that can not resolve name
Root name servers :
    * contacts authoritative name server if name mapping not known
    * Gets mapping
    * Returns mapping to local name server
    * There are 13 root name servers worldwide
TLD and Authoritative Servers
Top-level domain (TLD) servers: responsible for com, org, net, edu, etc, and all top-level country domains uk, fr, ca, jp.
    * Network solutions maintains servers for com TLD
    * Educause for edu TLD
Authoritative DNS servers: organization’s DNS servers, providing authoritative hostname to IP mappings for organization’s servers
    * Can be maintained by organization or service provider
Local Name Server
It does not strictly belong to hierarchy.
Each ISP (residential ISP, company, university)has one Also called “default name server”
When a host makes a DNS query, query is sent to its local DNS server


Network Topologies

Bus Topology

Bus networks (not to be confused with the system bus of a computer) use a common backbone to connect all devices.A single cable, the backbone functions as a shared communication medium that devices attach or tap into with an interface connector. A device wanting to communicate with another device on the network sends a broadcast message onto the wire that all other devices see, but only the intended recipient actually accepts and processes the message. Ethernet bus topologies are relatively easy to install and don't require much cabling compared to the alternatives. 10Base-2 ("ThinNet") and 10Base-5 ("ThickNet") both were popular Ethernet cabling options many years ago for bus topologies. However, bus networks work best with a limited number of devices. If more than a few dozen computers are added to a network bus, performance problems will likely result. In addition, if the backbone cable fails, the entire network effectively becomes unusable.


Bus Topology

Ring Topology

In a ring network, every device has exactly two neighbors for communication purposes. All messages travel through a ring in the same direction (either "clockwise" or "counterclockwise"). A failure in any cable or device breaks the loop and can take down the entire network. To implement a ring network, one typically uses FDDI, SONET, or Token Ring technology. Ring topologies are found in some office buildings or school campuses.


Ring Topology

Star Topology

Many home networks use the star topology. A star network features a central connection point called a "hub node" that may be a network hub, switch or router. Devices typically connect to the hub with Un-shielded Twisted Pair (UTP) Ethernet. Compared to the bus topology, a star network generally requires more cable, but a failure in any star network cable will only take down one computer's network access and not the entire LAN. (If the hub fails, however, the entire network also fails.)


Star Topology

Tree Topology

A tree topology joins multiple star topologies together onto a bus. In its simplest form, only hub devices connect directly to the tree bus, and each hub functions as the root of a tree of devices. This bus/star hybrid approach supports future expansion of the network much better than a bus (limited in the number of devices due to the broadcast traffic it generates) or a star (limited by the number of hub connection points) alone.


Tree Topology

Mesh Topology

Mesh topology introduces the concept of routes. Unlike each of the previous topologies, messages sent on a mesh network can take any of several possible paths from source to destination. (Recall that even in a ring, although two cable paths exist, messages can only travel in one direction.) Some WANs, most notably the Internet, employ mesh routing. A mesh network in which every device connects to every other is called a full mesh. As shown in the illustration below, partial mesh networks also exist in which some devices connect only indirectly to others.


Mesh Topology