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Difficulty: MediumNAT and PAT Implementation

An organization needs to connect 150 internal workstations on the 172.16.10.0/24172.16.10.0/24 private subnet to external web services simultaneously. The Internet Service Provider (ISP) has allocated only a single public IPv4 address (198.51.100.25198.51.100.25) to the perimeter router's WAN interface. Which address translation method must be configured on the router to meet these requirements by mapping multiple private IP addresses to unique source port numbers on the shared public IP address?

  1. Port Address Translation (PAT / NAT Overload)Answer
  2. B
    Static Network Address Translation (Static NAT)
  3. C
    Dynamic Network Address Translation (Dynamic NAT pool)
  4. D
    Layer 2 MAC Address Translation (MAT)

Answer

Port Address Translation (PAT / NAT Overload) must be configured on the router.
Port Address Translation (PAT), also referred to as NAT Overload, is designed specifically for scenarios where multiple internal hosts with private IPv4 addresses must share one or a few public IPv4 addresses. PAT achieves this by translating both the Layer 3 IP address and the Layer 4 TCP/UDP port numbers. Each outbound session from an internal workstation receives a unique ephemeral source port tied to the single public IP address (198.51.100.25198.51.100.25), allowing the router's translation table to correctly forward returning traffic to the originating host.

Step-by-Step Solution

1
Analyze internal host counts and external public IPv4 address availability
150 private hosts (172.16.10.0/24172.16.10.0/24) require internet access, but only one public IPv4 address (198.51.100.25198.51.100.25) is provided.
Because the ratio of internal hosts to external IP addresses is many-to-one, 1:1 mapping techniques cannot be used.
2
Evaluate mapping requirements at OSI Layer 3 and Layer 4
Multiplexing multiple private IP addresses over a single public IP address requires tracking connections using unique Layer 4 source port numbers (TCP/UDP).
Layer 4 port tracking allows the router to differentiate returning traffic for each of the 150 internal workstations.
3
Identify the standard technology used for many-to-one address translation
Port Address Translation (PAT), also known as NAT Overload, matches this exact functional capability.
PAT translates the private IP and original source port into the public IP and a unique allocated source port.

Key Concept

Port Address Translation (PAT / NAT Overload)
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Topics

NAT and PAT Implementation

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