NAT (Network Address Translation)

NAT translates private IP addresses ↔ public IP addresses, allowing multiple devices on a LAN to share a single public IP.

Why NAT?

• IPv4 exhaustion — not enough public IPs for every device.
• Private IPs (RFC 1918) are not routable on the internet.
• NAT sits at the router/gateway and translates addresses.

How NAT Works (Basic Flow)

LAN Device          Router (NAT)         Internet Server
192.168.1.10   →→→   203.0.113.1    →→→   93.184.216.34
(private IP)         (public IP)          (destination)

1. Device sends packet with src: 192.168.1.10
2. Router replaces src with public IP: 203.0.113.1
3. Server responds to 203.0.113.1
4. Router translates back and forwards to 192.168.1.10

Types of NAT

1. Static NAT

• One-to-one mapping — one private IP ↔ one public IP.
• Permanent, manually configured.
• Used for servers that need to be reachable from outside.

2. Dynamic NAT

• One-to-one but from a pool of public IPs.
• Private IP gets assigned a public IP from the pool temporarily.
• If pool is exhausted → connection fails.

3. PAT — Port Address Translation (Most Common)

• Also called NAT Overload or NAPT.
• Many-to-one — multiple devices share one public IP.
• Uses port numbers to distinguish connections.
• This is what your home router does.

PAT Example:

192.168.1.10:5001 → 203.0.113.1:5001
192.168.1.11:5002 → 203.0.113.1:5002
192.168.1.12:5003 → 203.0.113.1:5003

NAT Translation Table

Private IP	Private Port	Public IP	Public Port	Destination
192.168.1.10	4321	203.0.113.1	5001	93.184.216.34:80
192.168.1.11	4322	203.0.113.1	5002	93.184.216.34:80

NAT Advantages & Disadvantages

Advantages	Disadvantages
Conserves public IPs	Breaks end-to-end connectivity
Hides internal network (security)	Complicates P2P / VoIP / gaming
Easy LAN management	Adds latency (translation overhead)
No need to renumber LAN on ISP change	Stateful — NAT table can overflow

NAT Traversal

• STUN — discovers public IP/port
• TURN — relays traffic through a server
• UPnP — devices request port mappings automatically
• Port Forwarding — manually map external port to internal device

NAT vs PAT

	NAT	PAT
Mapping	IP to IP	IP+Port to IP+Port
Public IPs needed	One per device	One for all devices
Most common?	No	Yes (home routers)