Use Wireless Diagnostics on your Mac
If you can connect to your Wi-Fi router but have issues loading web pages, sending or receiving email or streaming music or video, use Wireless Diagnostics to analyse the network’s connection to the internet. After the analysis is complete, you get a list of detected issues and possible solutions and information about Wi-Fi best practices.
Note: Wireless Diagnostics doesn’t change your network settings.
On your Mac, quit all open apps.
Try to join the Wi-Fi network you’re having problems with (if you’re not already connected).
Press and hold the Option key, click the Wi-Fi status icon
in the menu bar, then choose Open Wireless Diagnostics.Follow the on-screen instructions to analyse your network connection.
After the analysis is complete, click the Info buttons
in the Summary pane to learn more about each item in the list.
Wireless Diagnostics also generates a compressed file that contains details that may be useful to your network administrator, service provider or IT support person.
The file is saved in the folder /var/tmp. The filename starts with WirelessDiagnostics and ends with .tar.gz.
To locate the file, click the Finder icon 
in the Dock, choose Go > Go to Folder, type /var/tmp, then press the Return key to open the folder that contains the diagnostics file.