NAT (Network Address Translation) and routing are both essential functions in networking that determine how data travels between networks, especially between local networks (LANs) and the internet. While they can operate together, they serve different purposes. Routing is responsible for directing traffic based on IP addresses, while NAT modifies IP address information in packet headers to allow private IP addresses to communicate over public networks.
Core Conceptual Differences
Routing
Routing is the process of selecting the best path for data packets to travel across interconnected networks. Routers examine the destination IP address in a packet, consult their routing table, and forward the packet toward its final destination through the most efficient route.
Key characteristics of routing:
- Works with both private and public IPs.
- Does not modify packet source/destination addresses.
- Uses protocols like RIP, OSPF, BGP to maintain route tables.
- Supports packet delivery across large internetworks like the internet.
NAT
NAT operates at the boundary of private and public networks. Its role is to translate private IP addresses (which are not routable on the public internet) into public IP addresses, allowing internal devices to communicate externally.
Key characteristics of NAT:
- Modifies source/destination IP addresses in packets.
- Used mainly for IPv4 to overcome address shortages.
- Often implemented in home/office routers.
- Types include Static NAT, Dynamic NAT, and PAT.
How They Work Together
In most home or small office networks, NAT and routing work together within a router. Here’s how:
- Routing decides which interface to send the packet to based on the destination IP address.
- If the packet is destined for the internet, NAT translates the private IP to a public IP before sending it out.
- On the return path, the router uses its NAT table to forward incoming packets to the correct internal device.
This interaction ensures private devices can access the internet using one public IP address, without exposing their internal IPs.
Example Scenario
Imagine a PC with IP address 192.168.1.10 sending a request to google.com:
- The router performs NAT and changes 192.168.1.10 to 203.0.113.2 (public IP).
- The router uses routing to forward the packet to the next hop toward Google’s servers.
- When the reply comes back, the router reverses NAT and delivers it to 192.168.1.10.
Main Differences Summarized
| Feature | NAT | Routing |
|---|---|---|
| Purpose | Translate private IPs to public IPs | Forward packets between networks |
| Alters IP Headers | Yes (IP address, possibly port) | No (just forwards based on destination) |
| IP Address Type | Private to public or vice versa | Any routable IP |
| Network Role | Used for internet access from private networks | Used in all network types (LAN, WAN, internet) |
| Visibility | Hides internal IPs from the internet | Exposes destination IP as-is |
Real-world Usage
Most modern routers perform both NAT and routing simultaneously. NAT is essential for IPv4 internet access when multiple devices share one public IP. Routing, on the other hand, ensures that packets reach their destination correctly, regardless of whether NAT is used.
In enterprise or carrier-grade networks, routing becomes far more complex, with dynamic protocols and hierarchical addressing, while NAT is often limited to edge firewalls or gateways.