Q My daughter and I travel internationally with our MacBook Pros, connecting to WiFi where we can, usually in hotels in which we are staying. Sometimes connections fail, according to the Network pane because the computer has a self-assigned IP address, although they have been configured to use DHCP. Trying to renew the DHCP lease fails, returning another self-assigned IP address. What is going wrong?
A Your Mac is correctly configured, and on each occasion connects successfully to the local wireless network. Having done so, it then looks for a DHCP server to provide it with an IP address, exactly as you have set up in the Network pane.
However the network DHCP server fails to provide an IP address, most probably because the server has run out of addresses, is broken, or operating under different rules, that is, it is incompatible. Because your Mac then lacks an assigned IP address, Bonjour cuts in and uses multicast DNS (mDNS) to self-assign the IP address that is shown in the Network pane. The network then lets you down a second time in failing to recognise that IP address, resulting in your connection being blocked.
When this happens, inform the management that their DHCP server is down or does not comply with RFC 2131, and threaten to inform sites on which they advertise their WiFi facilities that they are unreliable. Alternatively press them to provide you with an IP address that you can manually assign, that will give your Macs access to the network. The growing exception to this is when travelling in the Far East: because several of those countries have now exceeded their IPv4 allocations, they have switched to IPv6; make sure that this is set to Automatic in the Network pane just in case it is needed.
Updated from the original, which was first published in MacUser volume 26 issue 19, 2010.