All about NAT(NETWORK ADDRESS TRANSLATION)

NETWORK ADDRESS TRANSLATION

Have you ever notice that every computer and other communicating deceive in our organisation have private IP’s assign to it? But you still manage to surf web and communicate through internet despite of limitation that you need a valid public IP to communicate with internet.

How your private IP gets converted into Public IP?

All this wonder is done through the amazing NAT (Network Address Translation)

Network Address Translation:

NAT, define in RFC 1631, allows a host that does not have a valid registered IP address to communicate with other hosts on internet.

This capability of NAT to translate IP address of hosts have deeply influence majority of enterprises, as a result every enterprise IP networks today use private IP addresses for most hosts on the enterprise network and use a small block of public IP addresses, with NAT translating between the two.

 

NAT-Example

INSIDE LOCAL	INSIDE GLOBAL
10.1.1.2 10.1.1.3	200.1.1.2 200.1.1.3

NAT translate IP addresses inside a packet as it passes through a router.

NAT helps us into reducing the speed at which IPv4 address space is depleted.

Every enterprise has two pools of IP viz the private IP pool and the public IP pool. In NAT terms, these pools are known as INSIDE LOCAL and INSIDE GLOBAL respectively.

The NAT Router job is to convert inside local IP’s into inside global IP.

Depending upon how big the inside global IP pool is we have three different kinds of NATing

1. Static NAT
2. Dynamic NAT Without PAT
3. Dynamic NAT With PAT

Let’s understand each NAT one by one.

1. Static NAT:

Static NAT works just like the example in the above figure, but we need to statically mapped each IP addresses via configuration commands.

• A particular Inside Local address always maps to the same Inside Global (public) IP address.
• Static NAT does not conserve public IP addresses.

INSIDE LOCAL	INSIDE GLOBAL
10.1.1.2	200.1.1.2
10.1.1.3	200.1.1.3
10.1.1.4	200.1.1.4

It is one to one statically mapping of IP addresses.

2. Dynamic NAT Without PAT:

Dynamic NAT Without PAT, like static NAT, creates a one to one mapping between Inside Local to Inside Global address. However, unlike static NAT, it does so by defining a set or pool of Inside Local and Inside Global addresses. Which are mapped dynamically.

In Dynamic Nat, you are not sure which IP is going to get assigned to which host.

IP’s are mapped on first come first serve basis. If one host is assign one global IP then other host have to wait till that global IP is available again.

Criteria for Hosts to NAT:                                             NAT Pool:

10.1.1.0 – 10.1.1.255                                                           200.1.1.1

                                                                                             200.1.1.2

                                                                                             200.1.1.3

                                                                                             200.1.1.4

                                                                                             200.1.1.5

Inside local	Inside Global
10.1.1.5	200.1.1.1

3. Dynamic NAT With PAT:

As talked earlier, NAT is helping us in reducing the speed at which IPv4 address are depleting. NAT overloading, also known as Port Address Translation (PAT), is the NAT feature that actually provide significant savings of IPv4 addresses.

Through PAT we can assign a single global IP to many hosts. With PAT, instead of just translating the IP address, NAT also translate the port numbers as necessary. We can assign 6 concurrent hosts to one single global IP at one time.

PAT is very much like Dynamic Nat, but multiple local hosts can share a single public IP address by multiplexing using TCP and UDP port numbers.

Configuration of PAT is also as same as Dynamic NAT only additional command we use in PAT is “overloading”.

Criteria for Hosts to PAT:                                                   NAT Pool:

10.1.1.0 – 10.1.1.255                                                                200.1.1.1

                                                                                                  200.1.1.2

Inside local	Inside Global
10.1.1.5 10.1.1.6 10.1.1.1	200.1.1.1:1025 200.1.1.1:1026 200.1.1.1:1027

Different local IP’s are sharing same public IP. This is only achieved through PAT.

Hope this was informative if have any doubt or suggestion feel free to comment below