The Lazy Networker

NAT (Network Address Translation) allows private IP addresses to communicate with public networks like the Internet. PAT (Port Address Translation) is a form of NAT that uses port numbers to map multiple private IPs to a single public IP.

🔁 Types of NAT in Cisco

1️⃣ Static NAT (One-to-One)

• One private IP ↔ One public IP
• Used for servers (web, mail, VPN)

Configuration Example

interface g0/0
 ip address 203.0.113.2 255.255.255.252
 ip nat outside

interface g0/1
 ip address 192.168.1.1 255.255.255.0
 ip nat inside

ip nat inside source static 192.168.1.10 203.0.113.10

2️⃣ Dynamic NAT (Many-to-Many)

• Private IPs mapped to a pool of public IPs
• No port translation

Configuration Example

access-list 1 permit 192.168.1.0 0.0.0.255

ip nat pool PUBLIC_POOL 203.0.113.10 203.0.113.20 netmask 255.255.255.0

ip nat inside source list 1 pool PUBLIC_POOL

3️⃣ PAT (NAT Overload) – Many-to-One

• Multiple private IPs share one public IP
• Uses TCP/UDP port numbers
• Most common for Internet access

⚙️ PAT Configuration (Most Common)

🔹 Using Interface IP (Recommended)

access-list 1 permit 192.168.1.0 0.0.0.255

ip nat inside source list 1 interface g0/0 overload

🔹 Using Public IP Pool

ip nat pool PAT_POOL 203.0.113.50 203.0.113.50 netmask 255.255.255.0

ip nat inside source list 1 pool PAT_POOL overload

🔄 Inside vs Outside Interfaces (Mandatory)

interface g0/0
 ip nat outside

interface g0/1
 ip nat inside

📌 NAT Terms (Quick Reference)

🧪 Verification & Troubleshooting

show ip nat translations
show ip nat statistics
clear ip nat translation *
debug ip nat

🚦 Real-World Scenario (Home / Lab)

• LAN: 192.168.1.0/24
• ISP IP on g0/0
• Goal: Internet access for all LAN users

access-list 1 permit 192.168.1.0 0.0.0.255
ip nat inside source list 1 interface g0/0 overload

✔ This single command enables Internet for the entire LAN.

⚠️ Common Mistakes

❌ Forgetting ip nat inside / outside
❌ ACL mismatch (wrong subnet)
❌ NAT applied on wrong interface
❌ Missing overload keyword for PAT

🧠 CCNA / CCNP Exam Tips

• Static NAT → servers
• Dynamic NAT → limited public IPs
• PAT (Overload) → Internet access
• Order matters: Static NAT > Dynamic NAT > PAT