Question

Which type of network can be secured entirely through physical safeguards?

Answer 1

A physically isolated (air‑gapped) network. Because it has no external or wireless connections, access — and thus compromise — is only possible by physically accessing the systems, so physical controls (locks, guards, fenced rooms, controlled media) can in principle provide full protection.

Caveat: in practice you still need procedures/controls for removable media, maintenance, and insider threats, so logical and administrative safeguards are normally also used.