As there is a significant kernel upgrade as part of the 6 to 7 upgrade process, there is a possibility that your network interfaces may change name.
An indication of this is the upgrade will abort with this message on the screen
You need to switch to the second console by pushing Alt-F2 (which will automatically log in, no password needed) and manually update any network configuration that has changed.
When you do, you may see some PHP errors - this is expected, because the upgrade has not completed yet, and may be disregarded
At this point you should type 'ifup eth0' (or whatever your main interface was previously called) and look at the error
In this case, the interfaces have changed their location (and so, their name) so you will need to edit the files in /etc/sysconfig/network-script/ifcfg-ethX and move the contents around to match the new layout of the operating system. You can validate the settings with 'ifup ethX', and make sure that you can ping google.com (or equivalent). When you are happy with the changes, reboot the machine and the upgrade will proceed.