Yrurtj

EDIT / UPDATE:

firstly i was trying debian without desktop so that i can learn and spent my time on installing and playing with debian but as I was not able to connect to internet using command line so i plan to switch to desktop installation, because my 6 hours already gone just for connecting to the internet and both the solutions by dr01 sir and kais3n sir thought me a lot about networking in linux :)

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I just completed installing plain debian 8 (after 4 try) wihtout any desktop environment and is booting properly and I can able to login from terminal and also can do sudo.

so to install desktop and other stuffs I first tried sudo apt-get update but it says could not resolve 'security.debian.org' , then tried below:

  sudo ifconfig wlan0 up

  sudo iwlist wlan0 scan #getting all wifi name here..workss

  sudo iwlist wlan0 essid "YFy" enc "password123"

  sudo dhclient wlan0

all the above commands are executing without any error but again I am not able to update using sudo apt-get update command , its again saying could not resolve security.debian.org also tried ping google.com but its says unknow host google.com

my /etc/hosts :

 127.0.0.1 localhost.localdomain localhost

 127.0.1.1 noobdev



 #the following lines are desirable for IPv6 capable hosts

 ::1 localhost ip6-localhost ip6-loopback

 ff02::1 ip6-allnodes

 ff02::2 ip6-allrouters

and /etc/hostname:

 noobdev

It's a DNS problem; your machine is unable to resolve hostnames.

Specify the nameserver in the file /etc/resolv.conf by entering the following line on it:

nameserver 8.8.8.8

which is the IP of Google's public DNS server.

And you are sure that you get a IP Address after you run sudo dhclient wlan0.
You can check that with ifconfig.

If not you can try to set it manually if you know the IP Range of your WLAN.
Open the interface config with

   sudo nano /etc/network/interfaces

And change the following parameters matching to your config

  iface wlan0 inet static

    address 192.168.0.1 #the IP Address debian should get

    netmask 255.255.255.240 # your networkmask

    gateway 172.19.49.14 # Your gateway I guess your WIFI Router IP Address will work

After that you can set again your DNS manually in the /etc/resolv.conf like dr01 suggested

Please check if you have added the interface, with IP address provided by DHCP/router in /etc/network/interface. If not please add iface ethXXX inet dhcp.

You can also check here, looks like similar problem: https://ubuntuforums.org/showthread.php?t=1582015

Try to ping Debian ip from another device especially from AP or hotspot device. this work for me.