🌐 NAT (Network Address Translation)

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🧠 Top 5 Key Points

1. Translates Private IPs to Public IPs:
   NAT converts internal (private) IP addresses into a single or multiple public IPs so that devices inside a LAN can access the Internet.

2. Conserves Public IP Addresses:
   NAT allows multiple internal hosts to share one public IP, saving global address space.

3. Improves Security:
   Internal IPs remain hidden from the outside world, reducing attack exposure.

4. Types of NAT:

   • Static NAT: One-to-one mapping (internal ↔ public).

   • Dynamic NAT: Many-to-many mapping (pool of public IPs).

   • PAT (NAT Overload): Many-to-one mapping (most common).

5. Configured on Routers:
   NAT runs on routers at the boundary between private and public networks, translating IPs for outbound/inbound traffic.

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⚙️ Technical Summary

Feature	Description
OSI Layer	Network Layer (Layer 3)
Main Function	Translate IP addresses
Types	Static, Dynamic, PAT
Common Command	ip nat inside / ip nat outside
Use Case	LAN-to-Internet communication

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🧰 Packet Tracer Lab – NAT Configuration

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🎯 Goal:

Use PAT (NAT Overload) to allow multiple internal hosts to share one public IP for Internet access.

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🖥️ Network Topology

[PC1]---[Switch]---[Router]---[Cloud/Internet Router]

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IP Plan

Device	Interface	IP Address	Description
Router (R1)	G0/0	192.168.10.1	Inside (LAN)
	G0/1	200.0.0.1	Outside (Public)
PC1	NIC	192.168.10.10	Inside Host
PC2	NIC	192.168.10.20	Inside Host
ISP Router	G0/0	200.0.0.2	Public (Next Hop)

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🪜 Step-by-Step Configuration

🔹 1. Configure Router Interfaces

enable
conf t
interface g0/0
 ip address 192.168.10.1 255.255.255.0
 ip nat inside
 no shutdown
exit
interface g0/1
 ip address 200.0.0.1 255.255.255.0
 ip nat outside
 no shutdown
exit

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🔹 2. Create Access List for Inside Network

access-list 1 permit 192.168.10.0 0.0.0.255

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🔹 3. Configure NAT Overload

ip nat inside source list 1 interface g0/1 overload

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🔹 4. Configure Default Route

(To reach the internet via ISP)

ip route 0.0.0.0 0.0.0.0 200.0.0.2

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🔹 5. Verify NAT Configuration

show ip nat translations
show ip nat statistics

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💡 Testing NAT

1. On PC1 and PC2, set:

   IP Address: 192.168.10.10 / 192.168.10.20
   Subnet Mask: 255.255.255.0
   Default Gateway: 192.168.10.1
   

2. Ping from PC1 or PC2 → 200.0.0.2
   ✅ Should succeed.

3. On Router (R1), run:

   show ip nat translations
   

   You’ll see private IPs mapped to 200.0.0.1 (public IP).

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🧠 Learning Summary

Concept	Description
NAT Function	Translates private IPs to public
PAT	Many-to-one mapping (most used)
Inside Interface	Connected to LAN
Outside Interface	Connected to Internet
Verification	show ip nat translations

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💬 Teaching Tip

“NAT is like a receptionist — many employees (private IPs) talk to outsiders (public IPs) through one phone number (public IP), but the receptionist knows who’s who internally.”