It seems your network is not working, and yum can't reach it's repositories. Make shore you can ping google.com or similar !
So far as I can tell, I can use the internet, send email (as evidenced by this response), and ping google.com:
--- google.com ping statistics --- 10 packets transmitted, 10 received, 0% packet loss, time 9009ms rtt min/avg/max/mdev = 182.396/182.734/183.188/0.551 ms, pipe 2
It would seem to me that my network is working... so is there some specific configuration that needs to be checked?
Also, I should mention that YUM has worked fine up until this error, and I have not made any changes to my network configuration since installing CentOS.
Dave