Internet access and package installation behind a proxy server in Ubuntu

When there is a proxy server in between your system and server you have to specify the proxy address in all the applications that is intended to access internet such as firefox, bittorrent... Proxy servers are mainly deployed for subnets, for example a network inside a college is connected to internet using a proxy server (not technically, since there are gateways,routers...) to sensor and restrict the students internet activities, protect from unauthorized access... 

If you have to run any application behind a proxy server, you have to specify the proxy server address and the port number to the application that need access the internet. This is because the default port numbers used by the transport  layer protocols are changed by the proxy server. Squid is one among the important proxy programs that is prefered by network administrators. In the following cases please replace the proxy address and port number with the corresponding feilds applicable to you.

Setting Proxy address in Mozilla  firefox

Click on edit --> Preferences.

Select the Network tab under the advanced tab. Click on the Settings button. In the Network settings window select the manual configuration. Then enter the HTTP proxy and port on the current window. Save the settings...When you start browsing the firefox will automatically ask for authentication. Then you have to specify the username and password in the proxy server. 

Setting Proxy in Synaptic package manager

If you are using synaptic package manager as the package installer, You have to do some configuration before using apt package manager.

Click on the Settings --> Preferences

Choose the network tab and select manual proxy configuration. Set HTTP proxy and FTP proxy. Then enter their port numbers also. Click on the authentication button to set username and password.

Setting proxy for APT package manager
APT is a command line utility for installing packages. Probably many people are addicted to terminal do the following . . .

If you have no authentication for the proxy server then use the following command line. 

export http_proxy=http://192.168.0.2:3128
export ftp_proxy=http://192.168.0.2:3128

If you have username and password the use

export http_proxy=http://username:password@192.168.0.2:3128/
export ftp_proxy=http://username:password@192.168.0.2:3128/