Computer Network
A computer network is a group of computer systems and other computing hardware devices that are
linked together through communication channels to facilitate communication and resource-sharing
among a wide range of users. Networks are commonly categorized based on their characteristics.
One of the earliest examples of a computer network was a network of communicating computers thatfunctioned as part of the U.S. military's Semi-Automatic Ground Environment (SAGE) radar system.
In 1969, the University of California at Los Angeles, the Stanford Research Institute, the University of
California at Santa Barbara and the University of Utah were connected as part of the Advanced
Research Projects Agency Network (ARPANET) project. It is this network that evolved to become
what we now call the Internet.
Networks are used to:
Facilitate communication via email, video conferencing, instant messaging, etc.
Enable multiple users to share a single hardware device like a printer or scanner
Enable file sharing across the network
Allow for the sharing of software or operating programs on remote systems
Make information easier to access and maintain among network users
There are many types of networks, including:
Local Area Networks (LAN)
The computers are geographically close together (that is, in the same building).
Wide Area Networks (WAN)
The computers are farther apart and are connected by telephone lines or radio waves.
Metropolitan Area Networks (MAN)
A data network designed for a town or city.
Home Area Networks (HAN)
A network contained within a user's home that connects a person's digital devices.
Intranet
An intranet is basically a network that is local to a company. In other words, users from within this
company can find all of their resources without having to go outside of the company. An intranet can
include LANs, private WANs and MANs,
Extranet
An extranet is an extended intranet, where certain internal services are made available to known
external users or external business partners at remote locations.
Internet
An internet is used when unknown external users need to access internal resources in your network. In
other words, your company might have a web site that sells various products, and you want any
external user to be able to access this service.
VPN
A virtual private network (VPN) is a special type of secured network. A VPN is used to provide a
secure connection across a public network, such as an internet. Extranets typically use a VPN to
provide a secure connection between a company and its known external users or offices.
Authentication is provided to validate the identities of the two peers.
Confidentiality provides encryption of the data to keep it private from prying eyes.
Integrity is used to ensure that the data sent between the two devices or sites has not been tampered
with.
Benefits of networking
There are lots of advantages from build up a network, but the three big facts are-
File Sharing
From sharing files you can view, modify, and copy files stored on a different computer on the network
just as easily as if they were stored on your computer.
Resource Sharing
Resources such as printers, fax machines, Storage Devices (HDD, FDD and CD Drives), Webcam,
Scanners, Modem and many more devices can be shared.
Program Sharing
Just as you can share files on a network, you can often also share program on a network. For example,
if you have the right type of software license, you can have a shared copy of Microsoft Office, or
some other program, and keep it on the network server, from where it is also run.
Network Host
A network host (or simply referred to as a host) can be any computer or network device connected tothe computer network. This computer can be a terminal or a web server offering services to its
clients.
Network Protocol
A network protocol (or just referred to as protocol) is a set of rules and conventions that are
necessary for the communication between two network devices. For example, two computers on a
network can communicate only if they agree to follow the protocols.
The following are some of the most widely referred network protocols:
Internet Protocol (IP Address)
An Internet Protocol address (IP address) is a numerical label assigned to each device (e.g.,
computer, printer) participating in a computer network that uses the Internet Protocol for
communication. An IP address serves two principal functions: host or network interface identification
and location addressing. Its role has been characterized as follows: "A name indicates what we seek.
An address indicates where it is. A route indicates how to get there."
IP addresses are binary numbers, but they are usually stored in text files and displayed in human-
readable notations, such as 172.16.254.1 (for IPv4), and 2001:db8:0:1234:0:567:8:1 (for IPv6).
Types of IP Address
Private IP Address: A private IP address is the one that is assigned to a computer on the Local Area
Network (LAN). A typical example of private IP address would be something like: 192.168.0.4
Public IP Address: A public IP address is the one that is assigned to a computer connected to the
Internet. An example public IP address would be something like: 59.93.115.119
In most cases a computer gets connected to the ISP network using a private IP. Once a computer is on
the ISP network it will be assigned a public IP address using which the communication with the
Internet is made possible.
How to Find the IP Address of a Computer?
Finding your public IP is extremely simple. Just type “what is my IP” on Google to see your public IP
address displayed in search results.
In order to find your private IP, just open the command prompt window (type cmd in the “Run” box)
and enter the following command:ipconfig/all
This will display a long list of details about your computer’s network devices and their configuration.
To see your private IP address, just scroll down to find something as “IPv4 Address” which is
nothing but your private IP.
Hyper Text Transfer Protocol (HTTP)
Hypertext Transfer Protocol, abbreviated as HTTP, is a communications protocol used for the
transfer of information over the Internet. A client makes an HTTP request using a web browser to
which an HTTP response is sent from the server.
File Transfer Protocol (FTP)
The File Transfer Protocol provides a standard for transferring files between two computers on the
network. FTP is most widely used in carrying out upload/download operations between a server and
a workstation.Simple Main Transfer Protocol (SMTP)
The Simple Mail Transfer Protocol provides a standard for sending e-mails from one server to
another. Most e-mail systems that send mail over the Internet use SMTP to exchange messages
between the server.
Telnet
Telnet is a network protocol that allows you to connect to remote hosts on the Internet or on a local
network. It requires a telnet client software to implement the protocol using which the connection is
established with the remote computer.
In most cases telnet requires you to have a username and a password to establish connection with the
remote host. Occasionally, some hosts also allow users to make connection as a guest or public.
After the connection is made, one can use text based commands to communicate with the remote host.
The syntax for using the telnet command is as follows:
telnet <hostname or IP> port
WWW
The World Wide Web (abbreviated as WWW or W3, and commonly known as the Web) is a system
of interlinked hypertext documents accessed via the Internet. With a web browser, one can view web
pages that may contain text, images, videos, and other multimedia, and navigate between them via
hyperlinks.
It is the collection of internet resources (such as FTP, telnet, Usenet), hyperlinked text, audio, and
video files, and remote sites that can be accessed and searched by browsers based on standards such
as HTTP and TCP/IP.
SSH - Secure Shell
Developed by SSH Communications Security Ltd., Secure Shell is a program to log into another
computer over a network, to execute commands in a remote machine, and to move files from one
machine to another. It provides strong authenticationand secure communications over insecure
channels. It is a replacement for rlogin, rsh, rcp, and rdist.
SSH protects a network from attacks such as IP spoofing, IP source routing, and DNS spoofing.
An attacker who has managed to take over a network can only force ssh to disconnect. He or she
cannot play back the traffic or hijack the connection when encryptionis enabled.
When using ssh's slogin (instead of rlogin) the entire login session, including transmission of
password, is encrypted; therefore it is almost impossible for an outsider to collect passwords.SSH port forwarding
An SSH service that provides secure and encrypted connections to traditionally non-encrypted
services, such as e-mail or news.
SSH port forwarding allows you to establish a secure SSH session and then tunnel TCP connections
through it. It works by opening a connection to forward a local port to a remote port over SSH.
The client software (e.g. your e-mail client) is then set to connect to the local port. With SSH port
forwarding passwords are sent over an encrypted connection. Also called SSH tunneling.
Network Port
A computer may be running several services on it like HTTP (web server), SMTP, FTP and so on.
Each of these services are uniquely identified by a number called network port (or simply referred to
as port). If a computer wants to avail a specific service from another computer, it has to establish a
connection to it on the exact port number where the intended service is running.
For example, if a terminal is to request a web document from a remote server using HTTP, it has to
first establish a connection with the remote server on port 80 (HTTP service runs on port 80) before
placing the request.
In simple words, port numbers can be compared to door numbers where each door grants access to a
specific service on a computer.
List of Well-Known Ports
Port
Number Description
1 TCP Port Service Multiplexer
(TCPMUX)
5 Remote Job Entry (RJE)
7 ECHO
18 Message Send Protocol (MSP)
20 FTP -- Data
21 FTP -- Control
22 SSH Remote Login Protocol
23 Telnet
25 Simple Mail Transfer Protocol
(SMTP)
29 MSG ICP
37 Time
42 Host Name Server (Nameserv)
43 WhoIs
49 Login Host Protocol (Login)
53 Domain Name System (DNS)69 Trivial File Transfer Protocol (TFTP)
70 Gopher Services
79 Finger
80 HTTP
103 X.400 Standard
108 SNA Gateway Access Server
109 POP2
110 POP3
115 Simple File Transfer Protocol (SFTP)
118 SQL Services
119 Newsgroup (NNTP)
137 NetBIOS Name Service
139 NetBIOS Datagram Service
143 Interim Mail Access Protocol (IMAP)
150 NetBIOS Session Service
156 SQL Server
161 SNMP
179 Border Gateway Protocol (BGP)
190 Gateway Access Control Protocol
(GACP)
194 Internet Relay Chat (IRC)
197 Directory Location Service (DLS)
389 Lightweight Directory Access Protocol
(LDAP)
396 Novell Netware over IP
443 HTTPS
444 Simple Network Paging Protocol
(SNPP)
445 Microsoft-DS
458 Apple QuickTime
546 DHCP Client
547 DHCP Server
563 SNEWS
569 MSN
1080 Socks
A port number is a way to identify a specific process to which an Internet or other network
message is to be forwarded when it arrives at a server.
A port number is a way to identify a specific process to which an Internet or other network
message is to be forwarded when it arrives at a server. For the Transmission Control Protocol and
the User Datagram Protocol, a port number is a 16-bit integer that is put in the header appendedto a message unit. This port number is passed logically between client and server transport layers
and physically between the transport layer and the Internet Protocol layer and forwarded on.
Domain Name System
DNS, as it is called, refers to the hierarchical naming system used for computers, resources and
services on the Internet. It translates the computer hostnames to IP addresses.
DNS resolves an IP address to a hostname or vice versa.
DNS is basically a large database which resides on various computers that contains the names and IP
addresses of various hosts/domains. Other than ip-address DNS also associates various information
with the domain names.
Structure of a DNS
DNS uses a hierarchical tree based name structure. At top of the tree is the “root” (represented as a
dot (.) ) followed by the TLD ( Top Level Domain ), then by the domain-name and any number of
lower level sob-domains separated by a dot.
The Top Level Domains are divided into 2 categories:
1. Generic TLD (gTLD)
2. Country Code TLD (ccTLD)
Below are some of the common Generic Top Level Domains:
.com – comercial web sites
.org – non profit organizations web sites
.edu – restricted to schools and institutions.
.net – originally for network infrastructures, now unrestrictedBelow are some fo the Country Code Top Level Domains:
.us – United States
.in – India
.uk – United Kingdom
.ru – Russia
The following shows a sample representation of the structure of the DNS tree:
Authority, Delegation and Zone
The Authority for the root domain and gTLD lies with Internet Corporation for Assigned Numbers and
Names (ICANN). ccTLD’s are delegated to individual countries for administration purpose. Each
level in the hierarchy may delegate the authoritative control to the next lower level. There is a DNS
server running in every level of the hierarchy and the responsibility of running the DNS server lies
with the Authority at that level.
For Example, when the root domain gets a DNS query for www.example.com, the root will delegate
responsibility for resolving to its lower level “.com”, which in-turn will delegate to “example”.
Finally the DNS server in the “example” will respond with the IP address of the hostname “www”.
A zone is simply a portion of a domain. For example, the domain example.com may contain all the
information for a.example.com, b.example.com and c.example.com. However, the zone example.com
contains only information for example.com and delegates the responsibility to the authoritative name
servers for the subdomains. In general, if there are no subdomains, then the zone and domain are
essentially the same.
Resource Records
A DNS zone database is made up of a collection of resource records. Each resource record specifies
information about a particular object. The DNS server uses these records to answer queries for hosts
in its zone. For example, address mapping (A) record, map a host name to an IP address, and reverse-
lookup pointer (PTR) records map an IP address to a host name. Here are some of commonly used
Resource Records.A Record: The ‘A’ record specifies the IP address of a host. ‘A’ record will have the details of the
domain name and its associated IP address. When a Query is given to resolve domain name, DNS
server will refer the ‘A’ record and answer with the IP address present in the record.
PTR Record: A PTR record maps the IP address to a specific host.
NS Record: An NS record or name server record maps a domain name to a list of DNS servers
authoritative for that domain. Delegations depend on NS records.
MX Record: An MX record or mail exchange record maps a domain name to a list of mail exchange
servers for that domain. For example, when you send a mail to alpha@example.com, the mail will be
routed to the Mail Server as specified in MX record.
DNS Queries
A DNS query would be something like ‘what is the IP address of a.example.com’. A DNS server may
receive such a query for any domain, to which it has no information about. The DNS server will
respond is different ways for which it has no information about.
The following are the three types of DNS queries:
1. Recursive query
2. Iterative query
3. Inverse query
In Recursive query, the following are the steps involved when a host queries its local DNS server for
‘a.example.com’.
Host sends query ‘what is the IP address of a.example.com’ to locally configured DNS
server.
DNS server looks up a.example.com in local tables – not found
DNS sends query to a root-server for the IP of a.example.com
The root-server replies with a referral to the TLD servers for .com
The DNS server sends query ‘what is the IP address a.example.com’ to one of the .com TLD
servers.
The TLD server replies with a referral to the name servers for example.com
The DNS server sends query ‘what is the IP address a.example.com’ to name server for
example.com.
Zone file defines a A record which shows ‘a’ s IP address is x.x.x.x.
DNS returns the A record for ‘a’.
In Iterative query, if the DNS server doesn’t know the answer, it will refer other DNS server as
response. So the client which initiates the query will once again contact the DNS server which camein as response.
In Inverse query, an IP address will be provided and a hostname will be asked.
Proxy server
A proxy server is a server (a computer system or an application) that acts as an intermediary for
requests from clients seeking resources from other servers.
A client connects to the proxy server, requesting some service, such as a file, connection, web page,
or other resource available from a different server and the proxy server evaluates the request as a
way to simplify and control its complexity.
Proxies were invented to add structure and encapsulation to distributed systems.
Today, most proxies are web proxies, facilitating access to content on the World Wide Web and
providing anonymity.
Proxy Server - Types
There are many different types of proxy server and here are some common types:
Anonymous Proxy
Anonymous proxy servers conceal your information. When you go to request something from the
webpage, the webpage gets the IP address of the proxy server that you're using instead of your own.
The server has no way of accessing your IP address and communication between you and the proxy
server is encrypted, in cases where the greatest deal of security is desired.
High Anonymity ProxyThis type of proxy server does not identify itself as a proxy server and does not make available the
original IP address. High anonymity proxies, only include the REMOTE_ADDR header with the IP
address of the proxy server, making it appear that the proxy server is the client.
Transparent Proxy
An example of a transparent proxy would be a server that simply forwards your request to the
resource that you want without concealing any of your information. This may be used in the
workplace, where the IP address of the request is revealed to the server being requested from but
where the proxy provides access to the resource for a multitude of computers within the network.
Transparent proxies are generally not what people are looking for when they go shopping for proxy
server access online.
Reverse Proxy
A reverse proxy server is generally used to pass requests from the Internet, through a firewall to
isolated, private networks. It is used to prevent Internet clients from having direct, unmonitored
access to sensitive data residing on content servers on an isolated network, or intranet. If caching is
enabled, a reverse proxy can also lessen network traffic by serving cached information rather than
passing all requests to actual content servers.
No comments:
Post a Comment