my MBP didn’t connect to the internet anymore. airport could connect to my wireless router’s network but it used a self-assigned ip-adress (eg. 192.168.112.122) which is wrong. the error was in the system settings for the mac book pro’s firewall. if the firewall is set too restricted you might have clicked to block some program from network access which fetches the DHCP-lease ip-adress for your mac. simply turn the firewall off (‚allow all incoming connections‘) to see if it works. for me it did. bugs and bees…
the other problem was:
my mac1 couldn’t connect to my mac2 for afp-filesharing. mac2 could connect to mac1 for afp-filesharing. the problem was: on mac2 the firewall was set to ’set access for specific devices and applications‘ instead of ‚allow all incoming connections‘. so, same solution as above!