Ok, I already asked this in SATSQ, but he problem persisted even after I got it fixed.

So the issue is that totally out of the blue, sometimes in the middle of a browsing session, my connection to the Internet will fail. The status bar reads as fully connected, but my machine is apparently not receiving an IP address from the router and assigns itself one. This happens no matter what wireless network I'm on, and even an Ethernet cable doesn't work. The last time this happened (today, bringing the grand total to three) a friend and I were on YouTube when an error message came up and quickly vanished. I only caught that it said something like "IP address such-and-such is in use, changing to IP address so-and-so" At least that's how I remember it going. It showed up and then vanished before I could finish saying "oh sh**" After that, firefox gave the standard "Cannot find the server at http://www.youtube.com" while Safari gave me a "You are not connected to the Internet" message.

That's not all, however. When I rebooted, two windows popped up, one right after the other. The first one said "Would you like to allow the application 'configd' to accept incoming network connections?" with allow/deny buttons below it. The second one said the same thing, only it was asking about an application called 'mDNSResponder'. Now, being suspicious of programs that I've never heard of, I hit 'deny' on both of them and the problem is still here.

I'm very upset, this is the third time this problem has happened and I'm on the verge of marching down to the Mac superstore and demanding that they either fix the problem once and for all or give me a new computer, because this is just f***ing ridiculous. I called tech support twice. The first time they had delete some random files out of one of the core systems folders and it fixed it. Then about three days later it happened again. I took it into a Mac store and the tech there gutted the airport files and restarted it, and again that fixed it. That was Sunday and as of this morning the problem is back. If anyone can offer some meaningful advice, please help. And if you can help, just know that I have all the in-depth tech knowledge of a goldfish, so use simple terms please.

End of Line.

Is it possibly one of those rare Mac viruses?

If your IP addresses are all set to automatic you may need to manually set them as defined different addresses. I'm not an expert however and this is only a guess.

That was something I tried, thanks though. Inputting a manual IP address just caused he Internet to load endlessly. Good idea still.

Oh and just to cover all bases I've tried rebooting both the router and my computer, apart and together. Neither one worked

End of Line.

You definitely want configd and mDNSresponder to be able to accept network connections. Both are core networking utilities for OS X. And just in case it ever comes up, racoon also needs to accept network connections.

configd is the auto-configuration daemon that sits in between the Kernel Event Monitor (KEM) and the IP Monitor Agent (IPMA). Basically, it's what allows you to switch automatically between different network connections without having to go into the command line. configd is also the reason you end up with a self-assigned IP address, by the way. Basically, something is causing your network interface to go down, which causes the KEM to alert configd to a network configuration change, which causes configd to re-read the /Library/Preferences/SystemConfiguration/preferences.plist file and tell IPMA to rebuild the routing table and rebind the interfaces.

mDNSresponder is the zero-configuration networking (Bonjour) daemon.

racoon is the IPSEC daemon.

So as to your problem...what kind of router do you use? I know that some (for whatever reason) don't play nicely with non-Windows computers - mainly Linksys and Netgear variants, if I remember correctly. Second, under the "Network" system preference pane, are you using DHCP (it should either be shown directly on the main part of the pane for your interface, or in a tab (TCP/IP) in a drop down menu when you click the "Advanced" button)?

I ask because the "IP address such-and-such is in use, changing to IP address so-and-so" message you recall seeing implies that you were using a static IP address that the router then assigned to some other device. Which could very well trigger a network transition event in configd - and since there is no other address it can get, result in switching to a link-local self-assigned address.

Which also leads me to ask, since you indicate that this is both a wireless and wired router...do you have appropriate wireless security on the router, using WPA2 encryption? Or, in other words, do you know for certain what devices are connected to the network, since one seems to have kicked you off of it.

In any case, the issue seems to me to be devices not playing well together; either your computer and the router don't get along, or some other device is taking your IP address.

Thanks for beig so informative, in answer to all your questions:

What kind of router am I using?
A: an old PoS Actiontec (I think that's how it's spelled) if this is the source of the problem maybe it'll be enough to trash the stupid thing...

Am I using DHCP?
A: yes I am

Am I using WPA 2 encryption?
A: either WEP 64 bit or WPA (not specified if it was WPA 2 or not) both are available. According to our local tech expert and my computer, the Macs sign on with the WPA encryption. Also note, in my opinion both of our network passwords are very weak.

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Verizon FIOS by any chance?

Anyway, a quick google of Actiontec seems to indicate that it's not particularly well thought of for the most part, at least not as far as wireless goes. In your case, if it's both old and provided by your ISP, I'd try and get them to upgrade it. If it's just yours, then you might just want to upgrade it regardless (I'd personally suggest that having a wireless access point capable of supporting WPA2 is reason enough to upgrade...).

If the router is provided by your ISP, whether they upgrade you or not, you might just also want to consider having a separate wireless access point sitting between your computers and their router. It's what I use at home, myself. My computers, both wired and wireless, are connected to an Apple Time Capsule. The WAN port of the TC connects to the ISP cable modem. It lets me use a single IP address through the ISP, but share it on the internal network using DHCP and NAT.

I've also heard of people replacing the Actiontec as their primary router, but keeping it around as a client device...but I think that was primarily in cases where they needed the Actiontec specifically to connect to their Cable TV receivers to mediate TV programming updates...might not apply to you.