A router is performing PAT for inside users. Which detail allows multiple inside sessions to share one public IPv4 address at the same time?

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Port Address Translation (PAT), a form of Network Address Translation (NAT), enables multiple inside hosts to share a single public IPv4 address by differentiating sessions using transport-layer port numbers. When inside users initiate connections to external networks, the router modifies the source IP address to the public IP and assigns a unique source port number for each session. This port multiplexing allows the router to track and correctly forward return traffic to the appropriate inside host and application session. The key operational rule in PAT is that each inside session is uniquely identified by the combination of the public IP address and a transport-layer port number (TCP or UDP). This allows many simultaneous sessions to coexist even though they share the same public IP address. Cisco routers maintain a translation table mapping inside local addresses and ports to the outside global address and assigned ports, enabling precise session tracking and scalability beyond one-to-one NAT. A common exam trap is confusing PAT with static NAT or other translation methods that do not use port multiplexing. Static NAT creates a fixed one-to-one mapping between inside and outside addresses, which cannot support multiple simultaneous sessions on a single public IP. Understanding that PAT’s scalability depends on port number differentiation is critical for CCNA candidates, as this concept underpins many real-world Cisco NAT deployments and exam scenarios.

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