Definition:
NAT is a technique that allows a private network to communicate using a public IP address.

NAT Terminology

Let’s break down the flow with 3 example packets:

Packet 1 – Outgoing from LAN

Packet 2 – Response from outside

Packet 3 – From another LAN to NAT

Key to understand:

• Addresses on your LAN side = Local
• Addresses on the WAN side = Global
• If the address is on your router, it's Internal
• If it’s on the other side (remote), it’s External

Static NAT

Static NAT maps each Local Internal IP address to one specific Global Internal address.
Basically, it's a 1-to-1 translation.

Example Configuration

Commands:

SW0(config)# ip nat inside source static 192.168.10.10 209.165.201.1
SW0(config)# interface gig0/0
            ip nat inside
            no shutdown

SW0(config)# interface gig0/1
            ip nat outside
            no shutdown

After that, NAT needs to be applied to the correct interfaces: inside for LAN, outside for WAN.

To verify:

SW0# show ip nat translation

Dynamic NAT

Dynamic NAT lets multiple Local Internal addresses use a shared Global Internal address.
Not 1-to-1 anymore — here it's more flexible.

Example

Step-by-step

Step 1 – Create an ACL to match LAN traffic:

access-list 1 permit 192.168.1.0 0.0.0.255

Step 2 – Define a NAT pool:

ip nat pool NAT_POOL 209.165.201.1 209.165.201.2 netmask 255.255.255.0

Step 3 – Bind the ACL to the NAT pool:

ip nat inside source list 1 pool NAT_POOL

Assign NAT to interfaces:

SW0(config)# interface gig0/0
            ip nat inside
            no shutdown

SW0(config)# interface gig0/1
            ip nat outside
            no shutdown