Which NAT method allows multiple private IP addresses to be mapped to a single public IP address using different port numbers?
Correct Answer: C
Detailed Explanation: Port Address Translation (PAT), commonly known as NAT overload, is a technique that allows multiple devices on a private network to share a single public IP address by differentiating each connection through unique port numbers. PAT is an integral component of NAT (Network Address Translation) and is widely used to conserve public IP addresses, which are a limited resource. With PAT, when a device on a private network initiates a connection to an external network, the router translates the private IP address and port number into the public IP address along with a unique port number. This way, even though many devices share the same public IP, each connection remains distinct because the combination of the public IP and port number is unique. When responses are received, the router uses the port number to determine which internal device should receive the traffic. For example, consider a small office network where multiple computers need to access the Internet. Instead of requiring a unique public IP address for each computer, PAT allows all computers to share one public IP address. As each device establishes a connection, PAT assigns a different source port to each session. The router maintains a translation table that maps each internal private IP address and port to the corresponding public IP and port number. This table is used to properly forward returning traffic to the correct internal device. Static NAT and dynamic NAT are other forms of NAT. Static NAT creates a one-to-one mapping between a private IP address and a public IP address. Dynamic NAT, by contrast, maps private IP addresses to a pool of public IP addresses. Neither static nor dynamic NAT typically allows for multiple internal addresses to share a single public address simultaneously. PAT is unique in that it overloads a single public IP with many private addresses by using different port numbers. PAT is especially useful in environments where the number of available public IP addresses is limited. Its ability to multiplex several private connections over one public address is one of the primary reasons why PAT has become so ubiquitous in home networks, small businesses, and even larger enterprises where IP address conservation is a concern. In addition to conserving IP addresses, PAT also adds a layer of security by masking internal IP addresses from external networks. External hosts only see the public IP address and the port number, not the actual internal IP addresses. This makes it more difficult for attackers to target individual devices within a private network. Understanding PAT is essential for network professionals, especially for those preparing for the CCNA exam, as questions on NAT—particularly NAT overload—are common. Mastering the concepts behind how PAT operates, including the mapping of private IP addresses to public IP addresses using port numbers, is a key skill for troubleshooting and configuring networks. In summary, Port Address Translation (PAT) is the NAT method that allows multiple private IP addresses to share a single public IP address by assigning unique port numbers to each connection. This approach is vital for efficient IP address utilization and is a cornerstone of modern network address translation.
This CCNA practice question helps students prepare for Cisco networking certification exams by testing knowledge of network fundamentals, routing, switching, and network security concepts.