[CentOS] can NOT update CENTOS use YUM

We have CENTOS 5.1 on DELL server.  This server is OUTSIDE Firewall.  I tried to update it to latest release and get error.

any ideal?

=========================================
# yum update
Loading "fastestmirror" plugin
Loading mirror speeds from cached hostfile
Could not retrieve mirrorlist http://mirrorlist.centos.org/?release=5&arch=i386&repo=os error was
[Errno 4] IOError: <urlopen error (111, 'Connection refused')>
Error: Cannot find a valid baseurl for repo: base

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