What is NAT?
Network Address Translation translates the private addresses into public addresses before packets are routed to a public network. It allows a network device such as a router to translate addresses between the private and public network.
What are the Situations where NAT is required?
1. When we need to connect to the internet and our hosts don't have globally unique IP addresses.
2. When we want to hide internal IP addresses from outside for security purpose.
3. A company is going to merge in another company which uses same address space.
What are the advantages of Nat?
1. It conserves legally registered IP addresses.
2. It prevents address overlapping.
3. Provides security by hiding internal (private) IP addresses.
4. Eliminates address renumbering as a network evolves.
What are different types of NAT?
There are mainly three types of NAT:-
1. Static NAT
2. Dynamic NAT
3. Port Address Translation (Overloading)
What is Static NAT?
Static NAT allows for one to one mapping that is it translates one private IP address to one public IP address.
R1(config)# ip nat inside source static 10.1.1.1 15.36.2.1
R1(config)# int fa0/0
R1(config-if)#ip nat inside (It identifies this interface as the inside interface)
R1(config)# int fa0/1
R1(config-if)#ip nat outside (It identifies this interface as the outside interface)
In ip nat inside source command, we can see that the command is referencing the inside interface as source or starting point of the translation.
What is Dynamic NAT?
It maps an unregistered IP address to a registered IP address from out of a pool of registered IP addresses.
R1(config)# ip nat pool CCNA 190.1.1.5 190.1.1.254 netmask 255.255.255.0
R1(config)#ip nat inside source list 10 pool CCNA
R1(config)# int fa0/0
R1(config-if)#ip nat inside (It identifies this interface as the inside interface)
R1(config)# int fa0/1
R1(config-if)#ip nat outside (It identifies this interface as the outside interface)
R1(config)# access-list 10 permit 192.168.1.0 0.0.0.255 (To specify which unregistered addresses needs to be translated)
What is Port Address Translation (Overloading)?
It maps multiple unregistered IP addresses to a single registered IP address using different port numbers. PAT allows thousands of users to connect to internet using one public address only.
R1(config)# ip nat pool CCNA 190.1.1.5 190.1.1.254 netmask 255.255.255.0
R1(config)#ip nat inside source list 10 pool CCNA overload
R1(config)# int fa0/0
R1(config-if)#ip nat inside (It identifies this interface as the inside interface)
R1(config)# int fa0/1
R1(config-if)#ip nat outside (It identifies this interface as the outside interface)
R1(config)# access-list 10 permit 192.168.1.0 0.0.0.255 (To specify which unregistered addresses needs to be translated)
What are Inside Local, Inside Global, Outside Local, Outside Global address?
An Inside local address is an IP address of host before translation.
Inside Global address is the public IP address of host after translation.
Outside Local address is the address of router interface connected to ISP.
Outside Global address is the address of the outside destination (ultimate destination).
