Hi,
Does using a firewall have anything to do with network not working?
I use ipfw from the command line. The problem I'm facing is not being able
to achieve full duplex settings.
ifconfig shows full duplex active but even ping does not work. When I
bring the interface down,
/var/log/system.log shows interface comes up in half duplex.
And ifconfig shows
media : full-duplex ( <half-duplex> )
What could the problem be?
Is full duplex mode unsupported in OS X?
Where can I get help from?

Ipfw should not affect your duplex settings at all. Typically if there is a duplex negotiation problem, it's with the router. The computer simply does what it's told by the router.

Is it possible that this is a rounting table issue on this specific computer ? Just asking as this does happen from time to time on the Windows based PCs here at work and the symptoms are that some items will be able to connect, but not others... Just asking about this as it would seem that this is the only machine having this issue.

Very Similar Intermittent Issue
 

Ok, for weeks I've been struggling trying to figure out why I lose Internet access on my Mac Pro - not just Safari, but any web browser, Mail, Remote Desktop - I do retain access to any local resources however.

The particulars: Mac Pro, 10.4.9, all latest patches. Wired 100 Base T, static IP, no local firewall. Connected to a 100 Base T switch - switch connected to a Secure Computing firewall. Firewall is configured to allow complete access to the web from my static IP. Computer does not sleep; monitors does. DNS is set to the router; router DNS is from the cable company

When it works, it works fine. Every few days, I start to experience obvious slowdowns, then no access to the Internet. I have several other computer, both wired and wireless, and they all continue to access the internet without an issue.

The simplest fix for me is to unplug my ethernet cable from the back of the router, wait 10 seconds, and plug it back in - everything comes back fine until the next time.

I've tried both enet ports, and different patch cable and ethernet switch. I've tried different DNS, but it doesn't and shouldn't matter given the other computers' continued access.

I've not yet tried antoher use account - the problem is intermittment enough, and all my user stuff is of course unavailable if I play around in another account, so I'm wondering what I should look for in terms of logs?

I can fumfer around in Terminal if someone tells me what to do...

Thanks

1) Do you really lose access to the Internet, or just to the web? E.g. does email work?

2) Is the problem merely a DNS problem? You can test this by using IP addresses instead of host names. E.g. instead of trying http://www.apple.com
try: http://17.254.0.91