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Question

Difficulty: MediumInside Source NAT and PAT Configuration and Verification

An enterprise router is being configured to share a single public IPv4 address assigned to its WAN interface, Serial0/0/0, among private LAN devices matched by access list 105. What specific Cisco IOS keyword must be appended to the end of the global command `ip nat inside source list 105 interface Serial0/0/0` to enable Port Address Translation (PAT)?

Answer: overload

Answer

overload
The keyword `overload` enables Port Address Translation (PAT) on Cisco IOS routers. Appending `overload` to `ip nat inside source list <acl> interface <interface>` enables transport-layer port multiplexing so multiple inside local IP addresses can be translated to a single inside global IP address simultaneously.

Step-by-Step Solution

1
Analyze the global NAT configuration command syntax.
The statement `ip nat inside source list 105 interface Serial0/0/0` binds inside local traffic defined by ACL 105 to the WAN interface's public IP address.
Without port tracking enabled, dynamic NAT maps hosts on a strict 1-to-1 basis, exhausting the single IP address after one translation.
2
Identify the required keyword for enabling PAT (many-to-one translation).
Appending `overload` tells Cisco IOS to tracking source TCP/UDP port numbers, allowing multiple internal hosts to share the single IP address on interface Serial0/0/0.
The `overload` parameter explicitly instructs the router to perform Port Address Translation (PAT).

Key Concept

Port Address Translation (PAT) Interface-based Configuration Syntax
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Topics

Inside Source NAT and PAT Configuration and Verification

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