Network, troubleshooting

If you suddenly can't get to the internet, the best thing to do is to find out exactly what's broken first. Makes sense, right? So, a quick checklist of things:

1. run ifconfig and see the state of your interfaces; if a cable's come unplugged, you'll see that the interface isn't connected.
2. ping your gateway to see if it's your LAN that's down
3. ping something on the other side of your gateway (something in another subnet or outside your network, for example).
4. ping a known-pingable IP address on the internet. If you can ping 216.239.37.99 but not google.com, then you know something is wrong with your DNS. Other public places that respond to public pings are: cisco.com, yahoo.com, apple.com, google.com and freebsdwiki.net. It might not be a bad idea to write down the IP addresses of one or two of those places for future reference.
5. If it's been narrowed down to DNS, find out if it's your DNS or the DNS lookup itself that's causing a problem: try using dig or nslookup to do lookups against another DNS server.
6. ifconfig is your friend, unless it's not: if you've misconfigured the IP and netmask, you may not be able to connect to the machine. Remember to log in to the console or you can always connect a crossover cable and give your laptop/desktop that you've hooked up to it the gateway's IP -- this works when you've given your server a 32-bit netmask (255.255.255.255).