After installing an OS update, migrating your system, or otherwise performing a major change to your Mac, you may experience a problem where the system will no longer connect to the network, and upon checking your network settings it will show the system has a "self-assigned IP address." This may be the case even though other devices on the same network are working just fine. Additionally, while it may happen to some Macs, it may not happen to others that appear to be set up identically.
In attempting to fix this problem, people may try repairing permissions, booting to Safe Mode and running general maintenance routines, or even reapplying the latest Combo updater. While this may help some overcome the problem, others have tried this without success.
Even though it may seem like the problem is with the network settings on the system, the problem is more likely caused by an error in the system's firewall. In order for the DHCP client on the system to connect with your router and get an IP address, it must first establish a connection. If this connection is blocked, then the client will not receive a configuration and will instead configure itself for ad-hoc networking by assigning itself an IP address.
The firewall problems generally come from corruption in the firewall's settings, which is usually easily fixed by removing the settings and having the firewall set them up again. To do this, go to the /Macintosh HD/Library/Preferences/ folder and remove the file called "com.apple.alf.plist." Then reboot the system and log in again. After doing this you may see a few warnings about applications requesting to allow incoming connections, but once these are set up then your system should be back to normal again.