static IP & DHCP problems on LAN

[compdoc]

> so I was using the cable modem (which I normally use in "modem mode",
i.e., straight through to my own router)

Is this when you have the problem, or did the problem begin after you
connected everything back the way it was? 


>the machine (desktop & home "server") has gone off its static IP address
and onto a DHCP address from the router

That's the problem with static IPs - when there are changes to the lan, the
static settings have to be checked and changed manually if need be. IP
address conflicts are common using static, and can cause really odd
behaviors for the two PCs that are conflicting. With DHCP, you make a change
at the server when needed and all clients get updated. And no conflicts.

 
> which makes it impossible for me to SSH into it (unless I can guess the IP
address correctly)

Not knowing what you are specifically referring to, it's likely this can be
easily fixed. DNS might work.


>I also tried running the '/etc/init.d/networking restart' command today, &
that screwed up graphics & lightdm

That command is the one I've used for years across several versions of
Ubuntu with no issues. You saw this occur at the console?


>#auto ath0
>iface ath0 inet dhcp
>
>#auto wlan0
>iface wlan0 inet dhcp
>#v-
>
>I have no idea why it has ath0 and wlan0 stanzas --- I didn't type them in.
That machine has only ever had one RJ45 network connection.

Those seem to be wifi interfaces. If you have none, I would comment those
out. (disable) Maybe you had a wifi card at some point?