If your computer is connected to a local network that’s protected from the Internet by a firewall, you may need to specify proxy servers to access some Internet sites. A proxy server is a computer on a local network that acts as an intermediary between local computers and the Internet, to provide security, administrative control, and content caching.
You can also choose to bypass proxy settings for specific computers on the Internet (hosts) and segments of the Internet (domains) by adding the address of the host or domain in the “Bypass proxy settings for these Hosts and Domains” field.
To bypass specific hosts or networks by IP address, enter a specific IP address such as 192.168.2.11 or an address range, such as 192.168.2.* or 192.168.2.0/24.
Important: If you enter a numerical IP address or address range, you will only bypass the proxy when you connect to that host by specifying that numerical address, not when you connect to the host by a name which resolves to that address.
If you need to configure computers to bypass a proxy for several hosts by IP address, consider using a automatic proxy configuration (.pac) file. Note: When you use a .pac file, the hosts to bypass are specified in that file, and hosts listed in the Network preference pane are ignored.
For information about bypassing specific hosts or domains by name, search for "Entering proxy server settings" in Mac Help.