Which IPv6 address type should be assigned so the device is reachable only from the internal network?

Unique Local Addresses (ULAs) are designed for local network communication and are not routed on the global internet. They are comparable to IPv4 private addresses (RFC 1918) and ensure the device is reachable only within the internal network. Link-local addresses are for communication on a single link and are not routable beyond that link, making them too restrictive for an "internal network." Aggregatable global addresses are publicly routable and intended for internet communication, making them unsuitable for internal-only reachability. IPv4-compatible IPv6 addresses are a transition mechanism and not directly related to internal-only reachability.