Access Control
The access control list determines who can use this Charles instance. Normally you run Charles on your own machine and you only intend for yourself to use it, so localhost is always included in the ACL. However you may want to allow other hosts on your network to use Charles as their proxy; perhaps you are testing from multiple computers and want to view results back in your one instance of Charles.
You can enter individual IP addresses or IP addresses with subnet masks. You can enter subnet masks as an IP address followed by the number of mask bits, eg. 192.168.0.0/16 or as an IP address followed by a subnet mask, eg. 192.168.0.0/255.255.0.0.
The first time a host that is not in the ACL attempts to access Charles a warning message is displayed in Charles (as a popup message). The notifies you that the access attempt was blocked, shows you the IP address, and reminds you to add the IP address to the ACL if you want to allow access.
The warning is only shown once per IP address so that you don’t get swamped with warnings from repeated access.