Cannot execute sudo apt-get update

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http://askubuntu.com – I am new to Ubuntu and when I execute sudo apt-get update I get the following errors and warnings W: GPG error: http://ppa.launchpad.net quantal Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 5A9A06AEF9CB8DB0 W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/quantal-security/Release.gpg Something wicked happened resolving 'security.ubuntu.com:http' (-5 - No address associated with hostname) W: Failed to fetch http://extras.ubuntu.com/ubuntu/dists/quantal/Release.gpg Something wicked happened resolving 'extras.ub (HowTos)