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Inside Source NAT and PAT Configuration and Verification

46 questions

All types46Multiple choice24Multiple select10Short answer12
Question 41Question →

An administrator configures dynamic NAT on a Cisco IOS router using a pool containing a single public IP address (203.0.113.100203.0.113.100) to enable Internet connectivity for internal workstations. During testing, only the first internal user who initiates outbound traffic can access external sites. All subsequent attempts from other internal users to access the Internet fail. Which action on the router resolves this connectivity issue?

Show answer & explanation

Answer: Append the `overload` keyword to the `ip nat inside source list <ACL> pool <POOL>` configuration command.

Answer

Append the `overload` keyword to the global `ip nat inside source list <ACL> pool <POOL>` command to enable Port Address Translation (PAT).
When dynamic NAT is configured with an IP NAT pool containing a single public IP address, omitting the `overload` keyword restricts translations to a single 1-to-1 mapping. Once the first host initiates traffic, the single address in the pool is exhausted. Appending the `overload` keyword enables Port Address Translation (PAT), allowing multiple internal hosts to share the single IP address concurrently by tracking unique Layer 4 source port numbers.

Step-by-Step Solution

1
Analyze the symptom of dynamic NAT failure.
Without port translation enabled, dynamic NAT allocates pool IP addresses on a strict 1-to-1 basis. With a single public IP address in the pool, only one internal host receives a translation entry.
Determines why subsequent internal hosts fail to obtain an outbound translation.
2
Identify the required feature for many-to-one translation using a single IP address.
Port Address Translation (PAT) allows thousands of internal hosts to multiplex outbound connections over a single IP address using unique TCP/UDP source ports.
PAT is activated on Cisco IOS dynamic NAT configurations by appending the `overload` keyword.
3
Select the correct command modification.
Modifying the translation statement to include `overload` (`ip nat inside source list <ACL> pool <POOL> overload`) resolves the single-host limitation.
Applies PAT, enabling all internal hosts to share the single public IP address concurrently.

Key Concept

Port Address Translation (PAT) Overload Configuration
Question 42Question →

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)?

Show answer & explanation

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
Question 43Question →

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?

Show answer & explanation

Answer: Append the overload keyword to the statement ip nat inside source list 10 pool OUTSIDE_POOL.

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
Question 44Question →

A network administrator is configuring Port Address Translation (PAT) on a Cisco IOS router to allow hosts on the private subnet 172.16.20.0/24172.16.20.0/24 to access the Internet using an IP NAT pool named `PAT_POOL` with the public address 198.51.100.10198.51.100.10. Which TWO commands are required to successfully complete this dynamic PAT pool configuration?

Select all that apply

Show answer & explanation

Answer: ip nat inside source list 15 pool PAT_POOL overload; ip nat pool PAT_POOL 198.51.100.10 198.51.100.10 netmask 255.255.255.252

Answer

The configuration requires defining the dynamic IP NAT pool using `ip nat pool PAT_POOL 198.51.100.10 198.51.100.10 netmask 255.255.255.252` and referencing that pool in the NAT source translation statement with the `overload` keyword appended: `ip nat inside source list 15 pool PAT_POOL overload`.
Configuring PAT using a dynamic NAT pool requires defining the pool parameters via `ip nat pool <name> <start-ip> <end-ip> netmask <mask>` and linking an access list that identifies private traffic to that pool using `ip nat inside source list <acl> pool <name> overload`. The `overload` keyword enables multiplexing multiple internal sockets onto a single public address using port numbers.

Step-by-Step Solution

1
Define the public NAT address pool
Configured `ip nat pool PAT_POOL 198.51.100.10 198.51.100.10 netmask 255.255.255.252` to specify the public IPv4 address available for outbound translation.
A pool must exist before dynamic NAT/PAT mapping rules can reference it by name.
2
Bind the access list to the pool with port overload enabled
Executed `ip nat inside source list 15 pool PAT_POOL overload`.
The `overload` keyword instructs Cisco IOS to perform Port Address Translation (PAT), allowing multiple inside private hosts to share the single pool address by tracking unique TCP/UDP port numbers.

Key Concept

Port Address Translation (PAT) Pool Configuration Syntax
Question 45Question →

A Cisco router connects an internal LAN on interface GigabitEthernet0/0 (172.16.50.0/24172.16.50.0/24) to an ISP link on interface GigabitEthernet0/1 (198.51.100.2/30198.51.100.2/30). The network engineer issues the global command `ip nat inside source list 15 interface GigabitEthernet0/1 overload` to enable Port Address Translation (PAT). However, internal hosts cannot reach the Internet and no translations appear in the NAT table. The running configuration reveals the following interface settings:

text
interface GigabitEthernet0/0
ip address 172.16.50.1 255.255.255.0
!
interface GigabitEthernet0/1
ip address 198.51.100.2 255.255.255.252
ip nat outside

Which configuration command must be applied to interface GigabitEthernet0/0 to enable address translation?

Show answer & explanation

Answer: Apply `ip nat inside` under interface GigabitEthernet0/0 configuration mode.

Answer

Apply `ip nat inside` under interface GigabitEthernet0/0 configuration mode.
For Cisco IOS NAT/PAT to function, the router must know which interface represents the internal network and which interface represents the external network. The output shows `ip nat outside` on GigabitEthernet0/1, but GigabitEthernet0/0 is missing `ip nat inside`. Without this designation, packets entering GigabitEthernet0/0 are routed normally without triggering NAT evaluation.

Step-by-Step Solution

1
Analyze the given interface configuration for NAT boundary markers.
Interface GigabitEthernet0/1 is designated with `ip nat outside`, but GigabitEthernet0/0 lacks any NAT boundary designation.
Cisco IOS NAT routing logic requires packets to traverse from an interface designated as `ip nat inside` to one designated as `ip nat outside` (or vice versa) for translation rules to trigger.
2
Identify the required command to complete the NAT setup on the internal interface.
Configuring `ip nat inside` on GigabitEthernet0/0 establishes the internal NAT boundary.
This allows traffic originating from subnet 172.16.50.0/24172.16.50.0/24 entering GigabitEthernet0/0 to be evaluated by the `ip nat inside source list 15 interface GigabitEthernet0/1 overload` rule.

Key Concept

Inside and Outside NAT Interface Designation
Question 46Question →

A network administrator needs to configure static Network Address Translation (NAT) on a Cisco IOS router to map an internal server at private IPv4 address 10.20.30.510.20.30.5 to an external public IPv4 address 203.0.113.25203.0.113.25. Which global configuration command creates this static one-to-one inside source mapping?

Show answer & explanation

Answer: ip nat inside source static 10.20.30.5 203.0.113.25

Answer

ip nat inside source static 10.20.30.5 203.0.113.25
The command 'ip nat inside source static 10.20.30.5 203.0.113.25' correctly defines a one-to-one static mapping between an internal private IPv4 host address (inside local) and an external public IPv4 address (inside global).

Step-by-Step Solution

1
Identify the static NAT configuration command syntax in Cisco IOS.
The standard syntax is 'ip nat inside source static <local-ip> <global-ip>'.
Static NAT requires defining the parameter 'static' along with the inside local IP address followed by the inside global IP address.
2
Substitute the specified IP addresses into the syntax template.
Inside local IP is 10.20.30.5 and inside global IP is 203.0.113.25, giving 'ip nat inside source static 10.20.30.5 203.0.113.25'.
This establishes a permanent static translation entry between the internal host address and the public IPv4 address.

Key Concept

Static Inside Source NAT Configuration
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