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Difficulty: MediumInside Source NAT and PAT Configuration and Verification

A network engineer is troubleshooting an issue where only the first internal user from subnet 192.168.10.0/24192.168.10.0/24 can access external destinations at any given time. The Cisco IOS router configuration and verification command output are shown below:

text
ip nat pool OUTSIDE_POOL 203.0.113.50 203.0.113.50 netmask 255.255.255.248
ip nat inside source list 10 pool OUTSIDE_POOL
!
access-list 10 permit 192.168.10.0 0.0.0.255

text
Router# show ip nat statistics
Total active translations: 1 (0 static, 1 dynamic, 0 extended)
Hits: 412 Misses: 15
Expired translations: 12
Dynamic mappings:
-- Inside Source
access-list 10 pool OUTSIDE_POOL refCount 1

Which configuration change is required to allow multiple internal hosts to share the single pool IP address simultaneously?

  1. Append the overload keyword to the statement ip nat inside source list 10 pool OUTSIDE_POOL.Answer
  2. B
    Change access-list 10 to permit traffic from the public range 172.32.10.0 0.0.0.255 instead of the current subnet.
  3. C
    Configure ip helper-address 203.0.113.50 on the internal gateway interface to forward NAT translation requests.
  4. D
    Set an administrative distance of 210 on the dynamic NAT pool to ensure translated packets take priority.

Answer

Append the overload keyword to the statement ip nat inside source list 10 pool OUTSIDE_POOL.
Without the 'overload' keyword, Cisco IOS implements basic dynamic NAT, mapping inside local IP addresses to inside global IP addresses on a strict one-to-one basis. Because the defined NAT pool has only one IP address (203.0.113.50), only one internal device can translate its IP address at any given time, as confirmed by 'Total active translations: 1 (0 static, 1 dynamic, 0 extended)' in the output. Appending 'overload' converts dynamic NAT into Port Address Translation (PAT), allowing multiple inside hosts to share the single public IP address concurrently using unique source L4 port numbers.

Step-by-Step Solution

1
Analyze the existing router configuration and command output
The current command 'ip nat inside source list 10 pool OUTSIDE_POOL' without the 'overload' keyword performs standard 1-to-1 dynamic NAT mapping.
Because the pool OUTSIDE_POOL contains only a single IP address (203.0.113.50), standard dynamic NAT exhausts all available addresses after assigning it to the first host.
2
Identify the missing Port Address Translation (PAT) parameter
Adding 'overload' enables port-level multiplexing (PAT), creating extended translation entries that track source port numbers.
PAT allows thousands of concurrent internal hosts to share a single public IPv4 address simultaneously.

Key Concept

Port Address Translation (PAT) Overload Keyword Configuration
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Topics

Inside Source NAT and PAT Configuration and Verification

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