why can not get urllib2.urlopen?

Question

Why can't I get urlopen to work?

import urllib2

DOI = '10.1016/j.trf.2005.10.001'
link1 = "http://dx.doi.org/" + DOI  
print link1
arnumber = urllib2.urlopen(link1)
print arnumber

I receive this error:

URLError: <urlopen error [Errno 10060] A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond>

but my connection is true