Networks

A computer network is where two or more computers are connected by either physical kinks or wireless connections which have the ability to communicate and share resources with each other. To share resource and information.

Network topology

1. Bus topology
• Bus networks 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 messages.
• Ethernet bus topologies are relatively easy to install and do not require much cabling compared to the alternatives 10Base-2("ThinNet") and 10Base-5("ThickNet") both were popular Ether net cabling options many years ago for bus topologies. However bus networks work best with a limited number of devices. if more than a dew 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.
2. 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 building or school campuses.
3. Star topology
• Many home networks use the star topology. A star network features a central connection point called "hub node" that may be a network hub, switch or router. Devices typically connect to the hub with Unshielded twisted pair Ethernet.
• Compared to the bus topology, a star network generally requires more cable, but a failure in any star network cable will only tale down one computer's network access and not the entire LAN. (if the hub fails, however the entire network also fails.)
4. Tree topology
• Tree topologies integrate 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/start hybrid approach supports future expandability of the network much better than a bus or a star alone.

Wide Area Network(WAN)

• A Wide Area Network is a linked computer network distributed across a wide geographical location.
• You can have a number of different LAN's within the WAN (e.g. a company in New York might have a WAN to connect to its offices in Delhi)

OSI model

Layer	Info Transacted	TCP/IP Protocol
Application	Application message	HTTP, FTP, SNMP
Presentation	Compressed data
Session	Session messages
Transport	Multiple packets	TCP
Network	Packets	IP
Data Link	Frames	Ethernet, PPP
Physical	Bits	Wiring, cables

HTTP(Hyper Text Transfer Protocol)

The underlying protocol used by the World Wide Web. HTTP defines how messages are formatted and transmitted and what actions web servers and browsers should take in response to various commands.

FTP(File Transfer Protocol)

The protocol for exchanging files over the Internet. FTP works in the same way as HTTP for transferring Web pages from a server to a user's browser and SMTP for transferring electronic mail across the internet in that like these technologies, FTP uses the internet's TCP/IP protocols to enable data transfer

SNMP(Simple Network Management Protocol)

A set of protocols for managing complex networks. The first versions of SNMP were developed in the early 80s. SNMP works by sending messages called protocol data units (PDUs) to diffenrent parts of  a network SMNP compliant devices called agents store data about themselves in Management Information Bases and return this data to the SNMP requesters.

TCP(Transmission Control Protocol)

TCP is one of the main protocols in TCP/IP networks. Whereas the IP protocol deals only with packets. TCP enables two hosts to establish a connection and exchange streams of data. TCP guarantees delivery of data and also guarantees that packets will be delivered in the same order in which they were sent.

TCP/IP(Transmission Control Protocol/Internet Protocol)

the suite of communication protocols used to connect hosts on the internet. TCP/IP uses several protocols the two main ones being TCP and IP. TCP/IP is built into the UNIX operating system and is used by the internet making it the de facto standard for transmitting data over networks. Even network operating systems that have their own protocols such as Netware  also support TCP/IP

IP(Internet Protocol)

IP specifies the format of packets also called data-grams and the addressing scheme. Most netork combine IP with a higher level protocol called transmission control protocol (TCP) which establishes a virtual connection between a destination and a source.
IP by itself is something like the postal system, it allows you to address a package and drop it in the system, but there is no direct link between you and  the recipient. TCP/IP on the other hand establishes a connection between two hosts so that they can sent messages back and forth for a period of time.
The current version of IP is IPv4. A new version called IPv or iPng is under development.

Main Network Components

• Network Interface Card(NIC)
• Installed in a slot with a cable plugged into the back
• Plugged into a wall jack connection or into the hub/ switch directly
• Modem
• converts digital signals into analog form for transmission and incoming analog signals into digital signals across the telephone line.
• Hub and Switches
• Hub
• operates at physical layer
• acts as a connecting point
• passive, active and intelligent hub
• Switch
• offers direct connection to a particular PC
• Available for almost every OSI level
• Routers
• Operate at Internet Layer
• Evaluate network traffic and stop local traffic from causing congestion
• Filter out packets that need not be received
• Expensive and difficult to operate
• Gateways
• Special purpose computer allowing communications between dissimilar systems on the network.
• Operate at Application Layer primarily
• Difficult to install and configure
• Expensive
• Security of information system
• Firewalls
• A system designed to prevent unauthorized access to or from a private network. Firewalls can be implemented in both hardware and software or a combination of both. 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.
• In practice, many firewalls use two or more of these techniques in concert. A firewall is considered a first line of defense in protecting private information, For greater security data can be encrypted.
• Packet filter: Looks at each packet entering or leaving the network and accepts or rejects it based on user defined rules. Packet filtering is fairly addition, it is susceptible to I spoofing.
• Application gateway: Applies security mechanisms when a TCP or UDP connection is established. Once the connection has been made, packets can flow between the hosts without further checking.
• Proxy server: Intercepts all messages entering and leaving the network. The proxy server effectively hides the true network addresses.
• password protection
• Encryption
• The translation of data into a secret code. Encryption is the most effective way to achieve data security. To read an encrypted file, you must have access to a secret key or password that enables you to decrypt it. Unencrypted data is called plain text; encrypted data is refferred to as cipher text.
• There are two main types of encryption ; asymmetric encryotion and symmetric encryption.
• IP security
• Intrusion detection systems
• An intrusion detection system inspects all inbound and outbound network activity and identifies suspicious patterns that may indicate a network or system attack from someone attempting to break into or compromise a system.