NAT

Understanding Network Address Translation (NAT)

A Simple and Practical Guide for Networking Learners

The internet has grown far beyond what the original IPv4 addressing scheme was designed to support. With billions of devices now connected, IPv4 address exhaustion became a major challenge.

While IPv6 is the long-term solution, one technology has played a critical role in extending the life of IPv4:

Network Address Translation (NAT)

NAT allows multiple devices inside a private network to share a small number of public IP addresses when accessing the internet. This mechanism is widely used in home routers, enterprise networks, and ISP infrastructure.

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Why NAT Exists

Every device on the internet must have a unique public IP address. However, the IPv4 address space contains only about 4.3 billion addresses, which is far fewer than the number of devices connected today.

To solve this limitation, networks use private IP addressing internally and translate those addresses into public IP addresses when communicating with the internet.

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Private vs Public IPv4 Addresses

The concept of NAT depends on the difference between private and public IPv4 addresses.

Private IP Address Ranges

Defined by RFC 1918, these address ranges are reserved for internal networks:

Range	Address Space
10.0.0.0/8	10.0.0.0 – 10.255.255.255
172.16.0.0/12	172.16.0.0 – 172.31.255.255
192.168.0.0/16	192.168.0.0 – 192.168.255.255

Key Characteristics

• Private addresses can be reused by any organization
• They are not routable on the public internet
• ISPs discard packets containing private IP addresses
• They must be translated to public IPs using NAT before accessing the internet

Example:

Laptop IP: 192.168.1.10 (Private)
Router Public IP: 203.0.113.5

NAT translates:
192.168.1.10 → 203.0.113.5

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Cisco NAT Terminology (Important for Exams)

When working with NAT, Cisco defines four key address types. Understanding these is crucial for configuration and troubleshooting.

Term	Meaning
Inside Local	The private IP address of an internal host
Inside Global	The public IP representing the internal host
Outside Local	External host IP as seen from inside the network
Outside Global	The real IP address of the external host

Example

Internal PC: 192.168.1.10
Public NAT IP: 203.0.113.5
Web Server: 8.8.8.8

Type	Address
Inside Local	192.168.1.10
Inside Global	203.0.113.5
Outside Local	8.8.8.8
Outside Global	8.8.8.8

In most typical scenarios:

Outside Local = Outside Global

because the router usually translates only the source address, not the destination.

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Types of NAT

There are three main types of NAT used in networking.

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1️⃣ Static NAT

Static NAT creates a permanent one-to-one mapping between a private IP address and a public IP address.

When is it used?

Typically for servers that must be accessible from the internet, such as:

• Web servers
• Mail servers
• FTP servers

Example Mapping

192.168.1.10 → 203.0.113.10

Cisco Configuration

interface g0/0
ip nat inside

interface g0/1
ip nat outside

ip nat inside source static 192.168.1.10 203.0.113.10

Characteristics

✔ Manual configuration ✔ One-to-one translation ✔ Permanent mapping

2️⃣ Dynamic NAT

Dynamic NAT also provides one-to-one translation, but instead of manually assigning addresses, the router selects a public IP from a pool of available addresses.

How it works

• Internal hosts send traffic
• Router picks an available public IP from the pool
• Translation is created temporarily

If the pool runs out of addresses, new requests are dropped.

Cisco Configuration

Step 1 — Define internal addresses

access-list 1 permit 10.0.0.0 0.0.0.255

Step 2 — Create NAT pool

ip nat pool PUBLIC_POOL 203.0.113.10 203.0.113.20 prefix-length 24

Step 3 — Bind ACL to pool

ip nat inside source list 1 pool PUBLIC_POOL

Characteristics

✔ Automatic ✔ One-to-one translation ✔ Uses a public IP pool

3️⃣ Dynamic PAT (Port Address Translation)

This is the most commonly used NAT type today.

PAT allows many internal devices to share a single public IP address by using different port numbers.

Because of this, PAT is also known as:

NAT Overload

Example

Internal Host	Translated Address
192.168.1.10	203.0.113.5:1025
192.168.1.11	203.0.113.5:1026
192.168.1.12	203.0.113.5:1027

The port number uniquely identifies each connection.

Configuration Using Interface

access-list 1 permit 192.168.1.0 0.0.0.255

ip nat inside source list 1 interface g0/1 overload

Characteristics

✔ Many-to-one translation ✔ Uses TCP/UDP port numbers ✔ Most common NAT implementation

Essential NAT Verification Commands

When troubleshooting NAT on a Cisco router, these commands are extremely useful.

Show NAT Translations

show ip nat translations

Displays the active NAT table including:

• Inside local
• Inside global
• Outside addresses
• Port mappings

Show NAT Statistics

show ip nat statistics

Displays information such as:

• Total active translations
• NAT hits and misses
• Expired translations

Clear NAT Table

clear ip nat translation *

This removes all dynamic NAT entries.

Interface Configuration Requirement

For NAT to work correctly, interfaces must be defined properly.

Command	Purpose
ip nat inside	Interface facing the internal network
ip nat outside	Interface facing the ISP or internet

Example:

interface g0/0
ip nat inside

interface g0/1
ip nat outside

Important Note About ACLs in NAT

Many learners misunderstand the role of Access Control Lists (ACLs) in NAT.

In NAT configuration:

The ACL determines which traffic should be translated — not which traffic should be blocked.

It simply identifies the internal addresses eligible for NAT translation.

Quick NAT Revision Summary

NAT Type	Translation	Configuration	Use Case
Static NAT	One-to-one	Manual	Public servers
Dynamic NAT	One-to-one	Automatic pool	Limited public IPs
Dynamic PAT	Many-to-one	Uses ports	Home/enterprise internet access

Key Takeaway

Static NAT   → Manual one-to-one
Dynamic NAT  → Automatic one-to-one
Dynamic PAT  → Many-to-one (Overload)

Final Thoughts

Network Address Translation is one of the most important technologies in modern networking. Without it, the IPv4 internet would have run out of usable addresses long ago.

Even though IPv6 adoption is growing, NAT continues to play a major role in:

• Home networks
• Enterprise infrastructures
• ISP networks

For networking students and engineers, understanding NAT deeply is essential for troubleshooting, configuration, and exam preparation.

Keep learning, keep building networks. 🌐

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