The "Create UserDefinedNetwork" dialog has a problematic description under the subnet field. It gives example of 10.128.0.0/16 as a possible subnet. Unfortunately, this one is almost guaranteed to produce a broken CUDN, since this subnet is already used for the default cluster network. Use something like 192.168.123.0/24 instead.

The field also has a typo in its title, it should be "Subnet CIDR".

4.20

Always

1. Go to the Create UserDefinedNetwork dialog
2.
3.

It shows a bad example for subnet and has a typo

See the description