Introduction Computer Network

A computer network or data network is a digital telecommunications network which allows nodes to share resources. In computer networks, networked computing devices exchange data with each other using a data link. The connections between nodes are established using either cable media or wireless media.

Network

> Resource Sharing

> Centralized Sharing

>Security

Resource are available from only local PC, is called De-Centralized system

Network Model

> Client Server Model

> Peer to Peer Model

Email client :

In Internet, an email client, email reader or more formally mail user agent (MUA) is a computer program in the category of groupware environments used to access and manage a user's email.

Retrieving messages

A user's mailbox can be accessed in two dedicated ways. The Post Office Protocol (POP) allows the user to download messages one at a time and only deletes them from the server after they have been successfully saved on local storage. It is possible to leave messages on the server to permit another client to access them. However, there is no provision for flagging a specific message as seen, answered, or forwarded, thus POP is not convenient for users who access the same mail from different machines.

The mailbox storage can be accessed directly by programs running on the server or via shared disks. Direct access can be more efficient but is less portable as it depends on the mailbox format; it is used by some email clients, including some web mail applications.

Message composition

Email clients usually contain user interfaces to display and edit text. Some applications permit the use of a program-external editor.

The email clients will perform formatting according to RFC 5322 for headers and body, and MIME for non-textual content and attachments. Headers include the destination fields, To, Cc, and Bcc, and the originator fields From which is the message's author(s), Sender in case there are more authors, and Reply-To in case responses should be addressed to a different mailbox. To better assist the user with destination fields, many clients maintain one or more address books and/or are able to connect to an LDAP directory server. For originator fields, clients may support different identities.

Client settings require the user's real name and email address for each user's identity, and possibly a list of LDAP servers.

Submitting messages to a server

When a user wishes to create and send an email, the email client will handle the task. The email client is usually set up automatically to connect to the user's mail server, which is typically either an MSA or an MTA, two variations of the SMTP protocol. The email client which uses the SMTP protocol creates an authentication extension, which the mail server uses to authenticate the sender. This method eases modularity and nomadic computing.

Encryption

With no encryption, much like for postcards, email activity is plainly visible by any occasional eavesdropper. Email encryption enables privacy to be safeguarded by encrypting the mail sessions, the body of the message, or both. Without it, anyone with network access and the right tools can monitor email and obtain login passwords

All relevant email protocols have an option to encrypt the whole session, to prevent a user's name and password from being sniffed. They are strongly suggested for nomadic users and whenever the Internet access provider is not trusted

Encryption of the message body

There are two main models for managing cryptographic keys. S/MIME employs a model based on a trusted certificate authority (CA) that signs users' public keys. OpenPGP employs a somewhat more flexible web of trust mechanism that allows users to sign one another's public keys. OpenPGP is also more flexible in the format of the messages, in that it still supports plain message encryption and signing as they used to work before MIME standardization.

Webmail

In addition to the fat and small email clients, there are also Web-based email client. Webmail has several advantages, including an ability to send and receive email away from the user's normal base using a web browser, thus eliminating the need to install an email client on the user's device.

Like IMAP and MAPI, webmail provides for email messages to remain on the mail server.

Remote messages

POP3 has an option to leave messages on the server. By contrast, both IMAP and webmail keep messages on the server as their method of operating, albeit users can make local copies as they like. Keeping messages on the server has advantages and disadvantages

Advantages

Messages can be accessed from various computers or mobile devices at different locations, using different clients.

Disadvantages

With limited bandwidth, access to long messages can be lengthy, unless the email client caches a local copy.

Protocols

Popular protocols for retrieving mail include POP3 and IMAP4, sending mail is usually done using the SMTP protocol.

protocol use encrypt sessions plain text sessions only encrypt sessions only

POP3 incoming mail 110_pop3._tcp 995_pop3s._tcp

IMAP4 incoming mail 143_imap._tcp 993_imaps._tcp

SMTP outgoing mail 25 465

MSA outgoing mail 587_submission._tcp

HTTP Webmail 80 443

Different Networking Devices :

Different networking devices have different roles to play in a computer network. These network devices also work at different segments of a computer network performing different works. In our new series after network topology, we talk about different networking devices like a switch, router, hub, bridge etc.

In networking there some topology is used; are

Daisy chaining in computer networks Different network topologies Tree topology Mesh Topology Star Topology Ring Topology Bus topology

Networking devices:

Network Hub: Network Hub is a networking device which is used to connect multiple network hosts. A network hub is also used to do data transfer. The data is transferred in terms of packets on a computer network. So when a host sends a data packet to a network hub, the hub copies the data packet to all of its ports connected to.

Network Switch: Like a hub, a switch also works at the layer of LAN (Local Area Network) but you can say that a switch is more intelligent than a hub. While hub just does the work of data forwarding, a switch does ‘filter and forwarding’ which is a more intelligent way of dealing with the data packets.

Modem: A Modem is somewhat a more interesting network device in our daily life. So if you have noticed around, you get an internet connection through a wire (there are different types of wires) to your house. This wire is used to carry our internet data outside to the internet world.

Network Router: A router is a network device which is responsible for routing traffic from one to another network. These two networks could be a private company network to a public network. You can think of a router as a traffic police who directs different network traffic to different directions.

Bridge: If a router connects two different types of networks, then a bridge connects two subnetworks as a part of the same network. You can think of two different labs or two different floors connected by a bridge.

Repeater: A repeater is an electronic device that amplifies the signal it receives. In other terms, you can think of repeater as a device which receives a signal and retransmits it at a higher level or higher power so that the signal can cover longer distances.

IPV4 - IPV6 - APIPA

IPV4 :

Internet Protocol version 4 (IPv4) is the fourth version of the Internet Protocol (IP). It is one of the core protocols of standards-based internetworking methods in the Internet, and was the first version deployed for production in the ARPANET in 1983. It still routes most Internet traffic today,[1] despite the ongoing deployment of a successor protocol, IPv6. IPv4 is described in IETF publication RFC 791 (September 1981), replacing an earlier definition.

IPv4 is a connectionless protocol for use on packet-switched networks. It operates on a best effort delivery model, in that it does not guarantee delivery, nor does it assure proper sequencing or avoidance of duplicate delivery.

IPv4 uses 32-bit addresses which limits the address space to 4294967296 (232) addresses.

IPv4 reserves special address blocks for private networks (~18 million addresses) and multicast addresses (~270 million addresses).

IPv6 :

IPv6 is the next generation Internet Protocol (IP) address standard intended to supplement and eventually replace IPv4, the protocol most Internet services use today. Every computer, mobile phone and any other device connected to the Internet needs a numerical IP address in order to communicate with other devices. The original IP address scheme, called IPv4, is running out of numbers. Please see the Internet Society IPv6 Fact Sheet or our IPv6 Basics page for more information and background on IPv6.

APIPA :

(Automatic Private IP Addressing) The Windows function that provides DHCP autoconfiguration addressing. APIPA assigns a class B IP address from 169.254.0.0 to 169.254.255.255 to the client when a DHCP server is either permanently or temporarily unavailable. Designed for small non-routable networks, if a DHCP server becomes available later, the APIPA address is replaced with one from the DHCP server. For example, when a Windows Vista machine starts up, it waits only six seconds to find a DHCP server before assigning an IP from the APIPA range. It then continues to look for a DHCP server.

This IP address was assigned by APIPA, and the IPCONFIG utility reports the IP as an "Autoconfiguration IP address." See IPCONFIG.

TCP/IP Protocol

TCP/IP protocols map to a four-layer conceptual model known as the DARPA model , named after the U.S. government agency that initially developed TCP/IP. The four layers of the DARPA model are: Application, Transport, Internet, and Network Interface. Each layer in the DARPA model corresponds to one or more layers of the seven-layer Open Systems Interconnection (OSI) model.

Network Interface Layer:

The Network Interface layer (also called the Network Access layer) is responsible for placing TCP/IP packets on the network medium and receiving TCP/IP packets off the network medium. TCP/IP was designed to be independent of the network access method, frame format, and medium. In this way, TCP/IP can be used to connect differing network types. These include LAN technologies such as Ethernet and Token Ring and WAN technologies such as X.25 and Frame Relay

Internet Layer

The Internet layer is responsible for addressing, packaging, and routing functions. The core protocols of the Internet layer are IP, ARP, ICMP, and IGMP.

The Internet layer is analogous to the Network layer of the OSI model.

Transport Layer:

The Transport layer (also known as the Host-to-Host Transport layer) is responsible for providing the Application layer with session and datagram communication services. The core protocols of the Transport layer are Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP).

The Transport layer encompasses the responsibilities of the OSI Transport layer and some of the responsibilities of the OSI Session layer.

Application Layer

he Application layer provides applications the ability to access the services of the other layers and defines the protocols that applications use to exchange data. There are many Application layer protocols and new protocols are always being developed.

Wireless networking

A wireless network is a computer network that uses wireless data connections between network nodes.

Wireless networking is a method by which homes, telecommunications networks and business installations avoid the costly process of introducing cables into a building, or as a connection between various equipment locations.

Wireless telecommunications networks are generally implemented and administered using radio communication. This implementation takes place at the physical level (layer) of the OSI model network structure.

Examples of wireless networks include cell phone networks, wireless local area networks (WLANs), wireless sensor networks, satellite communication networks, and terrestrial microwave networks.

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