ip address & subnet mask !

The Subnet Mask is not an address per se, but a "blocker" of addresses you might say.

There is a way to set that in Terminal that I've seen, but I can't remember it.

If you want to change it, you could set Configure IPv4 to Manually and fill in the info, just being sure that the IP won't interfere or ever be the same as another IP.

Do you have Internet Sharing enabled somewhere perchance?

Are you sure that both these macs are getting IP addresses from the same place? (1 wired, 1 wireless...might mean that mac2 is getting neighbor's wifi)

Are you certain that this trouble mac is configured to be fully Automatic in the NetworkSystemPreferences?
"Conigure IPv4...Using DHCP" ( not manually, not dhcp with manual address, not bootP...)

Have you done a cold start to all your network devices (macs, router, modem, network printers?)
Power-cycle them off. Count 2 minutes. Start up modem, pause. Start up router/BaseStation, pause. Start up mac(s).

You haven't said what kind of router/baseStation you've got. 'Twould help, maybe.

added: Do you have a wireless modem/router AND a wireless airportBS/router/something)?

Message was edited by: Rick Van Vliet

Message was edited by: Rick Van Vliet

It would help if you reported the actual addresses you're seeing.

That's because if the address you're getting is 169.254.x.x then your machine isn't hearing a response from a DHCP server and is using a self-assigned address. The 169.254.x.x/255.255.0.0 block is specifically designed for this, therefore the subnet mask you're getting IS correct for your machine's network, it's just not on the network you expect/want it to be on.

You need to look at your network setup to see why the machine isn't getting a response from the DHCP server.

This happens to me constantly, and with no particular 'reason' why it occurs. I backed up my vaults in Aperture last night, left the machine doing it's thing and when I returned this morning, I am unable to access anything online. I have restarted router, modem, iMac...but have lost the note I had about how to "reset" all of the settings. Something like alt/opt/cmd/r/p? Holding all down at the same time while restarting. This sometimes resolves it, but it often takes four restarts before it actually kicks in.

Could someone please repost the "reset" key sequence? Thanks

Do you mean the PRAM Reset? CMD Option+pr
http://docs.info.apple.com/article.html?artnum=2238

Four of those in a row do an NVRAM Reset also.

Shouldn't affect your Networking though!?