Understanding NAT: Network Address Translation

In today’s networking environments, especially with the increasing number of devices requiring internet access, managing IP addresses efficiently is crucial. This is where NAT (Network Address Translation) plays a vital role. NAT is widely used in routers and firewalls from vendors like MikroTik, Cisco, Juniper, and others. It enables multiple devices on a local network to access the internet using a single public IP address. This process helps with both security and IP address conservation.

What is NAT?

Network Address Translation (NAT) is a method used in networking. It modifies IP address information in the IP packet headers. This modification occurs while the packets are in transit across a router or firewall. The primary purpose of NAT is to allow multiple private IP addresses to share a single public IP address. This occurs when accessing the internet.

Function of NAT:

NAT serves several key functions:

1. IP Address Conservation
   • NAT allows hundreds of devices with private IP addresses (like 192.168.x.x or 10.x.x.x) to connect to the internet using a single public IP.
2. Security
   • It hides internal network structures, making it more difficult for external attackers to directly target internal devices.
3. Routing Flexibility
   • NAT allows the internal addressing scheme to be independent of external networks, offering flexibility in network design.

How NAT Works:

Here’s a simplified explanation:

• A device on the LAN sends a request to the Internet (e.g., opening a website).
• The router (e.g., MikroTik or Cisco) modifies the source IP address of the packet from the private IP (e.g., 192.168.1.10) to the public IP (e.g., 203.0.113.5).
• The router keeps a translation table mapping private IPs and port numbers to the corresponding public IP and port.
• When the response from the internet arrives, the router refers to this table. It rewrites the destination IP back to the internal device’s private IP.
• The device receives the reply without needing to know that NAT occurred.

You can visit to learn more about the processing of NAT translation between Private and Public IP addresses. Check the diagram for details on how it translates from Private IP to Public IP.

NAT in Popular Technologies:

MikroTik Routers

• MikroTik uses masquerade, a type of dynamic NAT. It automatically handles IP changes on dynamic WAN interfaces (like PPPoE).
• Configuration involves setting a NAT rule in the firewall that targets the WAN interface and uses the “masquerade” action.

Cisco Routers

• Cisco supports several types of NAT: Static NAT, Dynamic NAT, and PAT (Port Address Translation), which is similar to masquerading.
• Configured via CLI using commands like ip nat inside and ip nat outside, along with ip nat translation rules.

Other Vendors

• Vendors like Juniper, Fortinet, and Ubiquiti offer similar NAT features. There are variations in terminology and interface, but the core functionality remains the same.

Types of NAT:

1. Static NAT – One-to-one mapping between private and public IP.
2. Dynamic NAT – Automatically assigns a public IP from a pool.
3. PAT (Port Address Translation) / Masquerade – Many-to-one mapping uses different port numbers. This method is most common in home and small business setups.

NAT is a foundational technology in networking that allows efficient use of IP addresses and adds a layer of security. Whether on a MikroTik, Cisco, or any other router, NAT ensures seamless internet access for internal devices. It prevents exposing them directly. Understanding NAT is essential for any network administrator or IT professional, as it underpins how most modern networks operate today.