This article explains about Harbor Network Requirements.
In a typical harbor installation, it requires 8 subnets at least to start. To customize the container network settings, choose the option Specify customized container network settings. You must specify at least one address pool base and size. The address pool is used when a Docker container starts. The Docker daemon (dockerd) will allocate an IP address to this container; the IP address that is assigned is selected from the address pool.
harbor-app/347a9782-20a2-41c5-a709-33598a28b464:~# docker network ls
NETWORK ID NAME DRIVER SCOPE
e490a451c407 bridge bridge local
888b4cc3a742 harbor-app_harbor bridge local
a1ba6eeedabd harbor-app_harbor-chartmuseum bridge local
447affff8491 harbor-app_harbor-clair bridge local
27bc9b2dbbe0 harbor-app_harbor-notary bridge local
fab440eac92e harbor-app_notary-sig bridge local
52205fc20dcb host host local
8795ca9c374f none null local
As the Harbor requires at least 8 subnets, the minimum Prefix size should be 27 and above. For more information on subnets, please see the below table: