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Network Address Translation

What is Network Address Translation (NAT)

- It is similar to Classless Inter-Domain Routing (CIDR) in that the original intention for NAT was to slow the depletion of available IP address space by allowing multiple private IP addresses to be represented by a much smaller number of public IP addresses.

NAT Names

- Local addresses: Are the ones we use before NAT translation. This means that the inside local address is actually the private address of the sending host that’s attempting to get to the internet.

- Outside Local address: would typically be the router interface connected to your ISP and is also usually a public address used as the packet begins its journey.

After translations inside local is then called the inside global address and the outside global address then becomes the address of the destination host.

Advantages and Disadvantages of NAT (Network Address Translation)


- Conserves Legally Registered Addresses

- Remedies address overlap events

- Increases Flexibility when connecting to the internet

- Eliminates address renumbering as a network evolves


- Conserves Legally Registered Addresses

- Translation results in switching path delays.

- Remedies address overlap events

- Causes loss of end-to-end IP traceability

- Increases Flexibility when connecting to the internet

- Certain applications will not function with NAT enabled

- Eliminates address renumbering as a network evolves

- Complicates tunneling protocols such as IPsec because NAT modifies the values in the header

List of Situations when NAT can be especially helpful:

- When you need to connect to the internet and your hosts don’t have globally unique IP addresses.

- When you’ve changed to a new ISP that requires you to renumber your network.

- When you need to merge two intranets with duplicate addresses.

Troubleshooting NAT issues:

- Check the dynamic pools. Are they composed of the right scope of addresses?

- Check to see if any dynamic pools overlap

- Check to see if the addresses used for static mapping and those in the dynamic pools overlap.

- Ensure that your access lists specify the correct addresses for translation

- Make sure there aren’t any addresses left out that need to be there, and ensure that none are included that shouldn’t be

- Check to make sure you’ve got both the inside and outside interfaces delimited properly

Types of Network Address Translation

Static NAT (one-to-One): Designed to allow one-to-one mapping between local and global addresses. This version requires you to have one real internet IP address for every host on your network

Dynamic NAT (Many-to-Many): Gives you the ability to map unregistered IP address to a registered IP address from out of a pool of registered IP addresses. When you have enough IP addresses for everyone who’s going to be sending packets to and receiving them from the internet at the same time.

Overloading (one-to-Many): This is the most popular type of NAT configuration. Understand that overloading really is a form of dynamic NAT that maps multiple unregistered IP addresses to a single registered IP address by using different source ports. This is also known as Port Address Translation (PAT) Allows you to permit thousands of users to connect to the internet using only one real global IP address.

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