Basic NAT Many-to-Many Exhaustion Scenario

The core concept tested here is the distinction between Basic NAT (many-to-many) and NAPT (many-to-one). The question explicitly states that RTA uses "NAT" with a pool of two public IPs for "many-to-many non-NAPT address translation."

Basic NAT performs only Layer 3 translation, mapping private IP addresses to public IP addresses on a one-to-one basis. It does not manipulate transport-layer port numbers. Consequently, each host requiring external access must be assigned a unique public IP from the pool. Since Host A and Host B are already occupying the two available public IP addresses, the pool is exhausted. When Host C attempts to initiate a connection, there are no free public IPs left to map its private address. Therefore, Host C's traffic cannot be translated and will be dropped.

Let's evaluate the distractors:

• Option A suggests RTA performs NAT based on the source port number. This describes Port Address Translation (PAT), also known as NAPT or Overload. The question specifically excludes this by stating "non-NAPT." Thus, this option is incorrect because the router is not configured to multiplex connections based on ports.
• Option C implies time-sharing or turn-based allocation. Basic NAT does not work this way; if a slot is occupied, it is unavailable until the existing session terminates or times out. There is no queuing mechanism described in standard basic NAT behavior.
• Option D suggests assigning an interface address. While some configurations allow using the outside interface IP as part of the pool, the question specifies a configured address pool. Even if the interface IP were available, the prompt implies the pool is the sole source. More importantly, without explicit configuration mentioning the interface IP, we assume the pool is strictly the two listed addresses. However, the primary failure mode is the exhaustion of the defined pool.

The most attractive wrong answer is A, as many engineers conflate NAT and NAPT. In modern networks, PAT (NAPT) is far more common than Basic NAT due to IPv4 scarcity. Test-takers might instinctively choose A because they know "routers usually use ports to save IPs," but the specific constraint "non-NAPT" invalidates that assumption.