My browser can’t connect to some sites

Your DNS (Domain Name System) server might be having problems or set to block access. In System Preferences’ Network pane, select your active connection then click Advanced. Switch to the DNS tab, make a note of existing settings in case you need to reinstate them, then delete them. Add these two entries: 8.8.8.8 and 8.8.4.4, which will set your Mac to use Google’s DNS servers.