A NAT (Network Address Translation) firewall works by translating private IP addresses used within a local network into a single public IP address for internet communication, while also acting as a security barrier to filter incoming and outgoing traffic.
How it works:
- IP Address Translation: Devices on a private network (e.g., 192.168.x.x) share one public IP when accessing the internet. The NAT firewall replaces the private IP in outgoing packets with the public IP and keeps track of connections to route responses back correctly.
- Traffic Filtering: By default, a NAT firewall blocks unsolicited incoming traffic from the internet since there’s no mapping for such requests. Only responses to outgoing connections or explicitly allowed traffic (like port forwarding) are permitted.
Example:
- A home network has multiple devices (phones, laptops) with private IPs (e.g., 192.168.1.2, 192.168.1.3). When a laptop accesses a website, the NAT firewall changes the source IP to the router’s public IP (e.g., 203.0.113.5) and tracks the connection. The website’s response is sent back to the public IP, and the NAT firewall routes it to the correct device.
- If an external attacker tries to send data directly to 203.0.113.5 targeting a random port, the NAT firewall drops it because no internal device initiated the connection.
In Cloud Environments:
For businesses using cloud services, Tencent Cloud’s NAT Gateway provides similar functionality. It allows private subnets in a VPC to access the internet securely while hiding internal IPs. Additionally, Tencent Cloud Security Groups and Network ACLs can work alongside NAT to enforce stricter traffic rules, blocking unauthorized access.
Example use case: A company’s cloud servers in a private subnet use Tencent Cloud NAT Gateway to download updates without exposing their private IPs. Incoming traffic is blocked unless configured via port forwarding or security rules.