On a machine with two Ethernet cards, where only one is connected and that one is enumerated as eth1 (eth0 is the unconnected one), the installer is not able to detect this and use the connected one; it tries to use the unconnected eth0 and behaves as if there was no connection at all: updates cannot be downloaded, network sources cannot be checked, and, especially, the second card is not even configured so after installation, the machine behaves as if it was not connected at all.