X.25 internetworking: which ITU-T standard provides virtual-circuit service for interconnecting multiple X.25 networks (i.e., gatewaying across packet-switched networks)?

Introduction / Context:
The X.25 suite defined packet-switched virtual circuits within a single public data network. When two such networks needed to interconnect and carry virtual circuits between them, an additional standard specified the interface and procedures at the network boundary. Knowing which document covers inter-network gatewaying clarifies the architecture of legacy data networks.

Given Data / Assumptions:

• Context: packet-switched public data networks using X.25.
• Goal: interconnect networks and carry virtual circuits across them.
• We select the standard addressing inter-network interfaces.

Concept / Approach:
X.75 defines the inter-network interface between X.25 networks, enabling virtual circuit transfer across network boundaries. X.25 itself specifies subscriber-to-network and internal procedures, X.21 covers circuit-switched signaling interfaces, and X.400 is a message handling system standard unrelated to virtual-circuit network interconnection.

Step-by-Step Solution:

Verification / Alternative check:
Historical PDN documentation references X.75 trunks between carriers to extend virtual circuits internationally, confirming X.75’s role.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming X.25 alone covers all aspects; it does not specify carrier-to-carrier interconnection, which is the realm of X.75.

Final Answer:
X.75.