Proper network address
I think proper here just means that the IP address in CIDR notation is the first address in the range specified by the IP address and its prefix.

This is the case for 192.168.23.0/24 and 10.57.13.64/28 and not the case for 193.205.16.132/26 (whose first address in the network range is 193.205.16.128) and 172.16.4.0/20 (whose first address is 172.16.0.0).