Edit the server-root/httpacl/gen*.proxy-admserv.acl files to add ip to the authentication list, in addition to user and group, as shown below:
acl "proxy-admserv"; authenticate (user,group,ip) { database = "default"; method = "basic"; };
Add the following access control entry:
deny absolute (all) ip !="ip_for_which_access_is_allowed";
For example:
acl "proxy-admserv"; authenticate (user,group,ip) { database = "default"; method = "basic"; }; deny absolute (all) ip !="205.217.243.119";