Q On several occasions, I have found myself able to connect to my local network via a weak WiFi signal, but was totally locked out via Ethernet, unable even to connect to my router. After following a suggestion for fixing WiFi problems, these have been solved. How does that work?
A This may well help anyone running any version of OS X and experiencing network connection problems.
The suggested solution effectively creates a new network connection linked to a named location, and renews the DHCP lease for it (assuming that you are not using fixed IP addresses, but have them assigned by a DHCP server).
To do this, open the Network pane in System Preferences, and using the Location popup menu, create a new location through the Edit Locations… command, then click on the + tool to add that new location. With that location selected, click on the Advanced button, then open the TCP/IP tab. Ensure that it is set to configure IPv4 using DHCP, and click on the Renew DHCP Lease button. When the IP address is assigned, click on OK.
This is not black magic, but a sound way of creating a new network setting which you can readily switch to through the Locations menu in the Network pane. As a new setting, it should discard any dross that could be causing problems with your existing settings, and start afresh. If you do not wish to create a new location, but fix the default network settings, you could get a similar result by trashing the network settings property list, restarting, and creating the settings from new.
IP addresses that are assigned by a DHCP server are not supposed to last forever, but to be handed out on lease. It is an old trick to repair a broken or difficult network connection by asking the server for a renewed lease, as that will sort out any systems whose lease has expired without renewal. If your devices are sitting long enough on a network to worry about renewing their DHCP leases, then you should really be asking yourself whether they should have fixed IP addresses, as detailed here.
Updated from the original, which was first published in MacUser volume 28 issue 18, 2012.