What is NAT?

01Why is NAT Needed?

IPv4 Problem:

• IPv4 = ~4.3 billion addresses
• 8+ billion devices worldwide
• Not enough!

Solution:

• Use private IPs in internal network
• Share single public IP with NAT

Internal (Private)           NAT          Internet (Public)
192.168.1.10  ────┐
192.168.1.11  ────┼───→ [Router] ───→ 203.0.113.5
192.168.1.12  ────┘      (NAT)

02Private IP Ranges

Reserved by RFC 1918:

These IPs are not routed on the internet!

03NAT Types

1. Static NAT (1:1)

Internal: 192.168.1.10 ←→ External: 203.0.113.10

• Fixed external IP for each internal IP
• Used for servers
• Expensive (needs many IPs)

2. Dynamic NAT

Internal IPs → [NAT Pool] → External IPs

• Dynamic assignment from IP pool
• Wait when pool is full

3. PAT (Port Address Translation)

192.168.1.10:5000 → 203.0.113.5:40001
192.168.1.11:5000 → 203.0.113.5:40002
192.168.1.12:5000 → 203.0.113.5:40003

• Most common type
• Single IP, different ports
• Also known as "NAT Overload"

04How PAT Works?

Outgoing Packet:

PC1: 192.168.1.10:3000 → google.com:443

Router writes to NAT table:
| Internal IP:Port | External IP:Port |
|------------------|------------------|
| 192.168.1.10:3000 | 203.0.113.5:40001 |

Outgoing packet:
203.0.113.5:40001 → google.com:443

Incoming Response:

google.com:443 → 203.0.113.5:40001

Router looks at NAT table:
40001 → 192.168.1.10:3000

Forwards to internal network:
google.com:443 → 192.168.1.10:3000

05NAT Limitations

1. Peer-to-peer difficulty - Two devices behind NAT
2. Some protocols problematic - FTP, SIP
3. Port limit - ~65,000 simultaneous connections
4. No end-to-end connection - IP changes

06Port Forwarding

Access server behind NAT:

Internet → Router (203.0.113.5:80) → Web Server (192.168.1.100:80)

Router rule:

• External port 80 → Internal 192.168.1.100:80

07Summary

• NAT = Private IP → Public IP conversion
• PAT = Multiple device sharing with ports
• Solution for IPv4 insufficiency
• Home routers use PAT