N10-009 Question 146
Single answerAddress translation: NAT, Port address translation (PAT)A small company has 45 employee workstations on a private IPv4 network using addresses in the 192.168.50.0/24 range. The ISP has provided only one public IPv4 address for Internet access. Users report that they can browse the web successfully, but an administrator notices that all outbound connections appear to come from the same public IP address when viewed from external websites. The administrator wants to confirm that this is expected and identify the technology making it possible. Which solution best explains this behavior?
- A
Static NAT maps each internal workstation to the same single public IP address without using port numbers
- B
PAT allows multiple internal hosts to share one public IP address by translating source ports as well as IP addresses
- C
Dynamic NAT assigns one public IP address per internal workstation from a pool, even if the pool contains only one address
- D
A default gateway forwards traffic to the ISP router, which removes the need for NAT or PAT
Show answer and explanation
Correct answer: B
Explanation
The correct answer is PAT. In a real-world small-office deployment with many internal devices and only one public IPv4 address, PAT is the standard solution. PAT translates many private source addresses to one public address by using unique source port mappings for each session. This is often referred to as NAT overload on network equipment. By contrast, static NAT is one-to-one and dynamic NAT is many-to-many from a pool, but still generally one public IP per active internal host. The behavior in the scenario, where external sites see the same public IP for all users, is normal when PAT is in use. This aligns with common networking practice and with RFC 1918 private addressing guidance, which requires translation for Internet access when private IPv4 addresses are used.
- A. Incorrect.
Incorrect. Static NAT creates a fixed one-to-one mapping between one private IP address and one public IP address. It is commonly used for servers that must be reachable from outside the network. It does not let many internal clients simultaneously share one public IP by differentiating sessions with port numbers. A candidate might choose this option because it includes NAT, but the scenario specifically describes many clients using one public IP at the same time.
- B. Correct.
Correct. Port Address Translation (PAT), also called NAT overload, allows many private hosts to access the Internet using a single public IPv4 address. The device tracks each session by translating source TCP or UDP port numbers, and if needed, ICMP identifiers, along with the source address. This is why external sites see the same public IP for multiple users while return traffic is still delivered to the correct internal host.
- C. Incorrect.
Incorrect. Dynamic NAT uses a pool of public addresses and typically creates a temporary one-to-one mapping between an internal host and an available public IP. If the pool had only one address, only one inside host could be translated at a time with standard dynamic NAT. It would not support dozens of simultaneous users in the way described. This is a common confusion between dynamic NAT and PAT.
- D. Incorrect.
Incorrect. A default gateway is necessary so hosts know where to send traffic destined for remote networks, but routing alone does not solve the public-to-private IPv4 translation problem. Without NAT or PAT, private RFC 1918 addresses are not routable on the public Internet. Someone might choose this option by focusing on basic packet forwarding rather than address translation requirements.