port

Network Address Translation (NAT) with Port Address Translation (PAT) is a crucial technique enabling multiple devices on a private network to share a single public IP address when communicating with the outside world. This is achieved by mapping internal private IP addresses and port numbers to the single public IP address, utilizing different port numbers to distinguish between connections from different internal devices. For instance, multiple computers within a home network can simultaneously access websites through the same public IP address assigned by the Internet Service Provider (ISP), with PAT differentiating each computer’s traffic using unique port assignments.

This technology is essential for conserving public IPv4 addresses, which are a limited resource. Before the widespread adoption of IPv6, it played a significant role in mitigating IPv4 address exhaustion. Furthermore, it provides a basic level of security by hiding the internal network structure and IP addresses from the public internet, making it more difficult for external entities to directly target individual devices within the private network. Its introduction represented a paradigm shift in network management, optimizing address space usage and bolstering security.

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A fundamental difference lies in how internal addresses are mapped to external ones. One method focuses on altering the source port number in addition to the IP address when translating from a private network to a public network. This allows multiple devices within the private network to share a single public IP address, with each device distinguished by a unique port. For instance, several computers browsing the web simultaneously can all appear to originate from the same public IP address, but each will use a different source port number.

This technique provides several advantages, primarily in conserving publicly routable IP addresses. In environments with a limited number of public IPs, it enables a larger number of internal devices to access the internet. This was particularly crucial in the transition to IPv4 exhaustion. It allows an organization to delay the expense and complexity of IPv6 deployment while still maintaining internet connectivity for its users. Security benefits are also realized, as internal network topology is effectively hidden from the outside world.

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