Error while installing NLTK dataset - nltk.download

I've ubuntu 14.04(64-bit) installed on my machine.

I'm trying to install NLTK supporting packages using the commands

import nltk
nltk.set_proxy(' ('USERNAME', 'PASSWORD'))
nltk.download()

But I'm getting the error:

[Errno -2] Name or service not known
False

Kindly help me to resolve this issue.

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