Ubuntu 12 crashed and took down network

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http://serverfault.com – We recently set up a new Ubuntu 12.04LTS server on our network. It's not fully configured so it's not doing much beyond sshd and a default apache2 install. But this evening it appears to have crashed. It wasn't responding to the network or the keyboard. But the worst part is, it took down the entire network. My knowledge of the network stack below OSI layer 3 is very limited, so the rest confuses me. When this machine was physically connected to the network, no other machine could connect to the outside internet. When things were broken, running arp showed that our gateway's IP address (HowTos)