1. CST 415 - Computer Networks
NAT
CST 415
4/10/2017
CST Computer Networks

2. CST 415 - Computer Networks
Topics
Problem Definition
NAT Types
NAT Addressing
Dynamic NAT
Overloading
4/10/2017
CST Computer Networks

3. Network Address Translation
Problem Definition
Network Address Translation
IPv4 addresses are becoming a rare commodity.
What is a simple way to extend the life of IP addresses until IPv6 becomes fully implemented?
Given a single IP address, map this single IP address onto multiple IP addresses.
4/10/2017
CST Computer Networks

4. CST 415 - Computer Networks
Problem Definition
How can we get something for nothing?
(e.g. How do we expand an address range without really adding more bits onto an address?)
4/10/2017
CST Computer Networks

5. CST 415 - Computer Networks
Problem Definition
The Internet sees a single point of presence –
The private sees a single point of presence as the router –
The NAT device translates from private network to external network.
4/10/2017
CST Computer Networks

6. CST 415 - Computer Networks
NAT
Developed by Cisco
NAT is an intermediary, very much like the old days when you would call an operator to “patch” a call through to you.
To do this, all you needed to know is to dial “0”, then request the other party you wished to speak to.
4/10/2017
CST Computer Networks

7. CST 415 - Computer Networks
NAT
As opposed to standard IP routing, NAT modifies the source and destination IP address in the IP header.
This modification is done based on mapping algorithms and tables.
4/10/2017
CST Computer Networks

8. CST 415 - Computer Networks
NAT types
Static NAT:
Mapping an unregistered IP address to a registered IP address on a one-to-one basis. Particularly useful when a device needs to be accessible from outside the network.
In static NAT, the computer with the IP address of will always translate to
4/10/2017
CST Computer Networks

9. CST 415 - Computer Networks
NAT types
Dynamic NAT:
Maps an unregistered IP address to a registered IP address from a group of registered IP addresses.
In dynamic NAT, the computer with the IP address will translate to the first available address in the range from to
4/10/2017
CST Computer Networks

10. CST 415 - Computer Networks
NAT types
Overloading NAT:
A form of dynamic NAT that maps multiple unregistered IP addresses to a single registered IP address by using different ports. This is known also as PAT (Port Address Translation), single address NAT or port-level multiplexed NAT.
In overloading, each computer on the private network is translated to the same IP address ( ), but with a different port number assignment.
4/10/2017
CST Computer Networks

11. CST 415 - Computer Networks
NAT types
Overlapping NAT:
When the IP addresses used on your internal network are registered IP addresses in use on another network.
The router must maintain a lookup table of these addresses so that it can intercept them and replace them with registered unique IP addresses.
The NAT router must translate the "internal" addresses to registered unique addresses as well as translate the "external" registered addresses to addresses that are unique to the private network.
This can be done either through static NAT or by using DNS and implementing dynamic NAT
4/10/2017
CST Computer Networks

12. CST 415 - Computer Networks
NAT types
Overlapping NAT:
The internal IP range ( xx) is also a registered range used by another network. Therefore, the router is translating the addresses to avoid a potential conflict with another network. It will also translate the registered global IP addresses back to the unregistered local IP addresses when information is sent to the internal network.
4/10/2017
CST Computer Networks

13. CST 415 - Computer Networks
NAT Example
The use of w1.x1.y1.z1 and w2.x2.y2.z2 represents valid public IP addresses as allocated by the Internet Assigned Numbers Authority (IANA) or an ISP.
If a private user at uses a Web browser to connect to the Web server at w2.x2.y2.z2, the user's computer creates an IP packet with the following information:
Destination IP address: w2.x2.y2.z2
Source IP address:
Destination port: TCP port 80
Source port: TCP port 1025
4/10/2017
CST Computer Networks

14. CST 415 - Computer Networks
NAT Example
The use of w1.x1.y1.z1 and w2.x2.y2.z2 represents valid public IP addresses as allocated by the Internet Assigned Numbers Authority (IANA) or an ISP.
This IP packet is then forwarded to the NAT protocol, which translates the addresses of the outgoing packet to the following:
Destination IP address: w2.x2.y2.z2
Source IP address: w1.x1.y1.z1
Destination port: TCP port 80
Source port: TCP port 5000
4/10/2017
CST Computer Networks

15. CST 415 - Computer Networks
NAT Example
The use of w1.x1.y1.z1 and w2.x2.y2.z2 represents valid public IP addresses as allocated by the Internet Assigned Numbers Authority (IANA) or an ISP.
The NAT protocol keeps the mapping of { , TCP 1025} to {w1.x1.y1.z1, TCP 5000} in a table.
The translated IP packet is sent over the Internet. The response is sent back and received by the NAT protocol. When received, the packet contains the following public address information:
Destination IP address: w1.x1.y1.z1
Source IP address: w2.x2.y2.z2
Destination port: TCP port 5000
Source port: TCP port 80
4/10/2017
CST Computer Networks

16. CST 415 - Computer Networks
NAT Example
The use of w1.x1.y1.z1 and w2.x2.y2.z2 represents valid public IP addresses as allocated by the Internet Assigned Numbers Authority (IANA) or an ISP.
The NAT protocol checks its translation table and maps the public addresses to private addresses and forwards the packet to the computer at The forwarded packet contains the following address information:
Destination IP address:
Source IP address: w2.x2.y2.z2
Destination port: TCP port 1025
Source port: TCP port 80
4/10/2017
CST Computer Networks

17. CST 415 - Computer Networks
NAT Example
For outgoing packets from the NAT protocol, the source IP address (a private address) is mapped to the ISP allocated address (a public address), and the TCP/UDP port numbers are mapped to a different TCP/UDP port number.
For incoming packets to the NAT protocol, the destination IP address (a public address) is mapped to the original intranet address (a private address), and the TCP/UDP port numbers are mapped back to their original TCP/UDP port numbers.
4/10/2017
CST Computer Networks

18. CST 415 - Computer Networks
NAT Example
Packets that contain the IP address only in the IP header are properly translated by NAT. Packets that contain the IP address within the IP payload may not be properly translated by NAT.
4/10/2017
CST Computer Networks