What is the difference between running Dependabot updates involving custom registries versus fully public dependencies?

For fully public dependencies, Dependabot's managed infrastructure can reach public registries directly with no special network configuration. When an update requires resolving against a private, internally-hosted registry (behind a corporate firewall or VPN, for instance), Dependabot needs a way to actually reach that network location, which isn't automatic the way public registry access is.

flowchart LR
    A[Dependabot managed infra] -->|Direct access| B[Public registry - npmjs, PyPI]
    A -->|Requires network path| C[Private internal registry]

GitHub addresses this specifically for private registries via credentials configuration (covered elsewhere) for registries reachable over the internet with authentication, or, for registries genuinely isolated on a private network with no public endpoint at all, GitHub Enterprise Server customers may need additional network configuration to bridge Dependabot's infrastructure to that internal network — a meaningfully more involved setup than simply pointing at a public package registry.