In your custom URL category, you can add only URLs or keywords, there is no option to add IP Subnets inside the URL category.
But you can insert subnet IP inside the custom URL but it will probably not work as you expect. Because Zscaler only looks for the URL and not for destination IP for what you inserted inside the custom URL field.
As an example, let say you add 220.127.116.11/16 in your custom URL category and created some policy for that category:
- this policy will not be triggered for servers 18.104.22.168 or 22.214.171.124 etc.
- this policy will not be triggered for URL http://example.com which is on the server under IP 126.96.36.199
- this policy will be triggered for URL http://188.8.131.52/16
- this policy will be triggered for URL https://app.184.108.40.206/16
- this policy will not be triggered for URL http://220.127.116.11/16
More information on what type of URLs formats Zscaler can accept, you can find it here: https://help.zscaler.com/zia/url-format-guidelines