Your question is a little hazy, but I think I know what you're asking. I'm going to assume that 1) your OS X machine is connected to a LAN through either a hub or a switch, and the hub is connected via an uplink port to a router, which connects either to DSL or a cable modem. I can't think of a way to keep your Mac connected to the LAN but not the interet, other than disconnecting the entire LAN from the internet. On the other hand, why would you need to? If you turn off file sharing, web serving, ftp and telnet, there's no real reason why you would want to "turn off" the Internet, is there?
If you still feel like your Mac might be vulnerable, you could install a firewall, or learn to configure the BSD firewall which is built into OS X.
Well, I have my beige G3 connected to a hub, along with an Airport base. I use Airport to network to my iBook, and share its modem with the iBook and the beige G3.
Problem is, just booting OSX or opening the Network control panel dials the modem. Why does it feel the need to do this?
As far as there being "no real reason to turn off the Internet", there's plenty of reason: If you have DSL or T1 its wonderful, but waiting for a modem to dial and connect (or potentially get a busy signal) is a pain. Also, if someone is using the phone at the time, it causes clicks in their ear, and you have to wait for a timeout to get control again.
Bottom line for modem users: *we* want to control when the modem dials, period! Airport doesn't allow this option.
Thanx for the reply, so I know I am not the only one.
Here in the Netherlands I use the sagem ISDN modem on one computer with the option "internet connection sharing on local network" and every time it connects the internet the phone will costs me about 5 cents (this is the connection cost for the phone company) and this multiple times a day.
I think there is now way it should connect to the internet if I don't want this.
It seems that I am experiencing the same problem as a few others.
At work, we use a WamNet MLP Software router to connect to the internet via 2 channel iSDN.
Normally this disconnects after a set period of inactivity (It is set at 3 minutes at present).
However when I am plugged in to the LAN with my iBook running MacOSX PB it stays connected constantly. I don't want to pay for two iSDN lines to be connected to the Internet for all the time that I am at work!
I can only presume that the OS must be making constant checks on the routers availability.
Has anyone logged this with Apple or got a fix for this?
I don´t know if this is the problem, but look if synchronizing with a timeserver is enabled.
If this is the case, MacOS X is trying to connect to a timeserver to get the current time on bootup. If you turn this off, It shouldn´t try to connect.
NTP is a pretty heavy-duty protocol -- if you want to cause your eyes to glaze over take a look at the docs included at /Library/Documentation/Administration/Services/ntp
It's designed to allow a server to keep an entire LAN in sync, thus in its default configuration it's pretty aggressive about nailing down the particulars of your machine's clock, how much it drifts, etc. While it's computing the drift (you can think of this as a calibration phase), it may make frequent connections to external time servers.
It could be argued that a full NTP implementation being on by default on a personal workstation is maybe overkill. Anyway, I'm having fun with it, as I sync all the machines at my house as well as the Linux, Solaris, and Windows NT machines under my desk at work to my little iMac at home.