Source: Mac Rumors forums In my situation, I had unplugged the ethernet cable from my wifi router and plugged it into my Mac.
My cable modem presumably didn't bridge the new MAC address, leading to the network problem. If these don't work, you might have a legitimate hardware issue.
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This area is also the part of Wireless Diagnostics you use to find the ideal spot for your router.
Change a router setting or location and see how it changed the performance.In crowded areas, these details help you find the best router to connect to. Keep this function open as you move to different areas.If you’re setting up a router yourself, this function lets you find the least crowded channel. You’re looking for the best signal under quality with the least amount of noise under signal.While configuration changes from migrating or restoring a system can lead to this problem, at other times major system crashes or power outages can do the same.Many cable modems bridge only the first MAC address they see.I have tried to manually enter all IPv4 information but doesn't help To reset the firewall, go to the /Macintosh HD/Library/Preferences/ folder and remove the file called "com.plist," and then restart your computer.After the system boots, you may be prompted to allow incoming connections to numerous programs and services, so accept these for now (you can always go to the Firewall settings and deny or remove entries later on) and then try connecting to the network again.These are some of the details listed when you hold down the option key, but you have a few more pieces of information here.The lists the DNS you are connected to and the wireless connection type, such as 802.11n.Now it is not getting connected to internet via Ethernet cable but Wireless works just fine.What happens is when I connect the Ethernet cable it says self assigned IP as shown in screenshot.