Practical motivations for NAT deployment: Which scenarios are valid reasons to run NAT on your edge router?

Introduction / Context:
NAT enables private networks to access public services without consuming large blocks of public IPv4 addresses. It also simplifies provider changes and allows overlapping address spaces to coexist during mergers.

Given Data / Assumptions:

• (1) Need Internet access but hosts lack globally unique addresses.
• (2) New ISP requires renumbering; you prefer to avoid touching all endpoints.
• (3) You want zero Internet access for any host.
• (4) You must merge two intranets using duplicate address ranges.

Concept / Approach:
NAT/PAT maps many private addresses to one/few public addresses, supports provider independence (translate to the ISP's address block), and resolves overlapping spaces via translation between internal domains. If the goal is to block Internet entirely, NAT is irrelevant—use security controls instead.

Step-by-Step Solution:

Verification / Alternative check:
Review typical NAT use cases: Internet access with RFC1918, provider migration, and overlap remediation (e.g., VRF/NAT-virtualization combos).

Why Other Options Are Wrong:
2 and 4 / 3 only / All of the above: Include either too few valid scenarios or include the invalid scenario (3).

Common Pitfalls:
Misusing NAT as a security control. NAT is not a firewall; deploy ACLs or stateful inspection for policy enforcement.

Final Answer:
1, 2 and 4