[Day 11] 5. Dynamic NAT with Overload

Definition: Dynamic NAT with overload, also known as Port Address Translation (PAT), allows multiple internal IP addresses to share a single public IP address by differentiating connections using port numbers.

How It Works?

1. Translation Table: NAT maintains a translation table that maps internal IP addresses and ports to a single public IP and unique port numbers.
2. Outgoing Connections: When an internal device initiates a connection, NAT translates the internal IP and port to the public IP and a unique port.
3. Incoming Responses: Responses to the public IP and port are translated back to the original internal IP and port using the translation table.

Example

In this example, two internal devices (172.16.10.2 and 172.16.10.3) are using a single public IP address (202.208.220.1) to access an external server (75.1.1.1).

• 172.16.10.2 initiates a connection using port 2000. NAT translates this to the public IP with port 3000.
• 172.16.10.3 connects using port 2001, which NAT translates to the same public IP but with port 3001.

The NAT device keeps a translation table to manage these connections, allowing both internal devices to communicate externally without IP address conflicts. This method enables efficient usage of public IPs while supporting multiple simultaneous connections.

• Internal Devices:

  • 172.16.10.2 uses port 2000 → 202.208.220.1 (public IP) with port 3000
  • 172.16.10.3 uses port 2001 → 202.208.220.1 with port 3001

• Connections:

  • 172.16.10.2:2000 → 202.208.220.1:3000 → 75.1.1.1
  • 172.16.10.3:2001 → 202.208.220.1:3001 → 75.1.1.1

• Scalability: Up to 65,535 internal devices can use the same public IP through different port numbers.

This allows efficient use of a limited number of public IP addresses while supporting many devices.

A(config)#ip nat inside source list 15 pool sunbeam overload

overload

For single IP NAT Configuration

• Access List Configuration:

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    access-list 50 permit 172.16.10.0 0.0.0.255

    
    Creates an access list (50) that permits traffic from the internal network (172.16.10.0/24) for NAT translation.

• NAT Pool Definition:

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    ip nat pool cdac 202.208.220.1 202.208.220.1 netmask 255.255.255.0

    
    Defines a NAT pool named "cdac" with a single public IP address (202.208.220.1) and its subnet mask.

• NAT Configuration:

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    ip nat inside source list 50 pool cdac overload

    
    Configures NAT to use the defined pool for the internal IPs specified in access list 50, allowing multiple internal addresses to share the single public IP.

  • Alternative Command:

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    ip nat inside source list 50 int g0/1 overload

    
    Instead of using a pool, this command translates addresses using the public IP of interface
    Copy

    g0/1

    , also supporting overload.

Monitoring Command

A#show ip nat translations