When performing the steps to setup SNMP on a XenServer Host as per the document here
http://support.citrix.com/article/CTX116187You get the error saying its unable to contact http:/// xxxxxxxxxxx.xxxxxxxxxxxx.com when running the yum command
You may need to manually add name servers to the config file.
To do this follow these steps using a text editor such as vi
vi /etc/resolve.conf
add the following lines
nameserver IP address of nameserver
Save and exit out of vi
Rerun the yum command as per the Citrix article above