Q: Ethernet has a self-assigned IP address and will not be able to connect...

I had the exact same problem.  I've been doing a lot of traveling and usually leave the MacBook Pro on, but this time I shut it off before going to the airport for the last leg of a trip home.  When I got home and booted up, I couldn't get a (real) IP address on either the Airport or the ethernet connection, even though other devices on the same network had no problem.  One caveat, though - be sure to turn off (and delete?) the Airport and ethernet connections in Network Preferences before deleting /Library/Preferences/SystemConfiguration/com.apple.airport.preferences.plist and /Library/Preferences/SystemConfiguration/com.apple.network.identification.plist .  Otherwise the files will be regenerated with the old (incorrect?) data when you shut the system down.

This is with Mac OS X 10.5.8 on a (late 2008) MacBook Pro.

To reiterate, this is what I did:

   Go to SystemPreferences->Network, in the left column highlight Airport, then at the bottom of the column pull down the tools list and select "Make Service Inactive".  Do the same for "Ethernet" if it's giving you problems.  Then delete both Airport and Ethernet.

Open a terminal window,

   cd /Library/Preferences/SystemConfiguration

   sudo rm com.apple.airport.preferences.plist

   sudo rm com.apple.network.identification.plist

Reboot

After reboot, go to SystemPreferences->Network, add Airport and Ethernet to the services, set up for DHCP, then everything works (for me).

When visiting family we had to manually assign IP addresses to one MacBook for both ethernet and airport connections, while the other one worked fine with regular DHCP. Now that we are back home, they both work fine with DHCP (after removing the manually set DNS servers).

If it crops up again, I'll try deleting the network services and plist files as suggested.

Yet another user experiencing this problem, on a MacBookPro running 10.6.8. First get the message that there is no IP address, then that I am using a self-assigned IP. I have tried disabling passive FTP mode and the approach outlined by WaltzKing above, both to no avail.

Interestingly enough, I dual boot a Linux distro, for which Ethernet also used to work by simply plugging in, and  now it no longer works there, either (actually a bigger problem for me, as this is my only means of connecting to the internet under this OS at the moment). I am almost totally clueless when it comes to network protocols, but I mention this issue about  Linux in hopes that it might add something to the discussion (namely, perhaps there could also be a hardware aspect to this issue?)

After experiencing all of the same issue described "self assigning ip", i spent hours trying to resolve.  I resolved my issue by simply going into my router 192.168.1.1 and changing the SSID name from the default.  Then I redirected my macbook to connect to this newly named network.  Issue resloved.   Hope that helps someone.

How I fixed the problem on my MacBook Pro.

I was suddenly experiencing this issue with any network I ventured onto. This was never a major problem for me because I would simply look at the network settings assigned to my iPhone and manually set the network up on my laptop using the same settings along with a different IP.

I remembered that the issue began for me right after I was experimenting with the security/firewall settings in the System Prefs and had enabled 'Stealth Mode' (System Prefs > Security > Firewall > Advanced).

Once I disabled Stealth Mode and set the Firewall to 'Allow all incoming connections', the problem was solved. When I enabled Stealth Mode again, I was presented with a dialog asking if I wanted to allow incoming connection from 'mDNSResponder'. I selected 'Allow', and now everything is back to normal including the use of Stealth Mode.

Hope this helps someone.

I have this issue as well with 10.6.8. I have tried EVERYTHING and nothing seems to make my internet steady. I was having no issues with  wireless, but now that I am using wired internet, everytime I stream video the connection cuts shortly after. Sometimes restarting the computer will allow me back onto the net for a short time, but as soon as I stream video the problem occurs again.

Does anyone have any new suggestions? Does anyone know what exactly is going wrong?

I have a PowerBook G4 that had this problem for several days. I had a "self-assigned IP address" that I suspect was the cause of the problem. I called apple who told me to restart the router... pah. I tried most of the suggestions in this thread, but none worked.

I decided to log into a guest account to see how the network settings compared. When I logged in, the PowerBook initiated setting up network preferences and it assigned a new IP address (beginning with 192 as opposed to 16 something). I logged back into my administrator account and I had access!

I recommend creating a guest account and logging in!

I have the same issue with my MacBook. Wifi works, but my wired ethernet port won't get an IP address from DHCP and self assigns 169.x.x.x. After reading this thread and getting mine to work, here's my guess at what's happening. There is poor communication between my mac and the dhcp server when the address up for renewal is already taken, eventually timing out. Maybe the Mac doesn't deal well with rejection or the dhcp server isn't good about suggesting an alternative. Apparently Windows PCs, as much as I dislike them, are more tolerant about this sort of thing. This can be resolved by configuring the DHCP sever to assign a fixed IP address based on the Mac's MAC address. This is what I did, and it worked like a snap. Manually configuring would have worked too, but this way I don't have to worry about DNS servers changing, etc. If you don't have the ability to change the server's config, other resolutions might include some of the recipes described in this thread, which amount to various ways of erasing the Mac's memory of what address it wanted to renew.

So I tried a lot of what was said in this post and nothing worked until I tried to log into the router (192.168.1.1 on a Linksys router) and instantly it said I was connected. What's weird is it wouldn't actually connect to the router, but I had internet access immediately. I have no idea why this worked, but it did for me. I was fixing a 13 in Macbook Pro running Snow Leopard 10.6.8. Hope this helps someone else.

This did it for me:

http://support.apple.com/kb/TS1920

I'm having the same issue on different computers. I had it on my son's 21 in iMac and my daughter's Macbook 13" pro, I am also encountering it on the Wifi of my 27in Imac, but still able to connect with ethernet.

When my son's workstation first encountered the issue, I tried a plethora of solutions found on the internet (including this support site), When I turned off the security, my son was able to connect without issue, but no one can live like that for long, so I wiped his hard drive and reinstalled system software. So now it connects without issue. My daughter's connected at first, now doesn't. I am not able to reinstall her system software and the same with my iMac.

There needs to be a solution that is consistant and works. C'mon Apple!

Hi Finally after struggling for two days found the solution. Took help of the mac helpline. Try this out.

Open Network preferences--> Advanced-->wifi

under preferred networks, delele the entire list by clicking on the '-' (minus) sign

Select ok--> apply

Try to reconnect

will ask for the password

Type in the password ans select ok

This shoud definetley work if deleting from keychain, trying from different user log-in, renewal of DHCP lease etc did not work

Hi Finally after struggling for two days found the solution. Took help of the mac helpline. Try this out.

Open Network preferences--> Advanced-->wifi

under preferred networks, delele the entire list by clicking on the '-' (minus) sign

Select ok--> apply

Try to reconnect after restarting mac

will ask for the password

Type in the password and select ok

This shoud definitely work if deleting from keychain, trying from different user log-in, renewal of DHCP lease etc did not work

ok...finally, changing the channel from the router, to channel 1, has solve the problem...now i get signal continnously...thnxs!

I tried what seems like everything, but here's the secret: First, restart your Mac. Then, Open network preferences by clicking the wifi symbol in the top right of the screen> select "Wi-Fi" > click "assist me..." at the bottom of the box.

Assist Me... > Diagnostics... > Wi-fi> select your wireless network> then WITH THAT OPEN, restart your router. You can try to power cycle it, but I just reset mine, it seemed easier at the time.

Then your Diagnostics box should say something like, "There are no problems detected" and you should be good to go!