Tuesday, January 19, 2016

Lab 2-2 Port Address Translation

Lab pre-requisites:
Lab 2-1 Packet Tracer Topology Download.

Topology Diagram

Connecting a LAN to the Internet requires a little thought as there are few ways of doing it. Here we are going to explore three of them:
  • Static NAT (one-to-one translation)
  • Dynamic NAT (many-to-many translation)
  • Dynamic NAT Overload or PAT (many-to-one translation)
Task 1: Defining static IP addresses and setting a static default route.
Task 2: Configure NAT.
Task 3: Configure PAT.

Port Address Translation
(NAT Overload)

Both, Dynamic NAT and PAT (Port Address Translation) are not used to allow users on the Internet to access local resources. They are designed to allow local users using private address scope to communicate on the Internet.

PAT is arguably the most popular way of connecting private network to the Internet. Before we do that let's clean up our previous lab configuration first:

On Branch Router:

Branch(config)#no ip nat inside source list 1 pool PUBLIC
Branch(config)#no ip nat pool PUBLIC netmask


Now we're good to go with PAT configuration.

Since all is already configured except for nat command let's add it (line in blue)

interface FastEthernet0/0
ip address
ip nat inside
duplex auto
speed auto
interface FastEthernet0/1
ip address
ip nat outside
duplex auto
speed auto

ip nat inside source list 1 interface FastEthernet0/1 overload
ip route
ip flow-export version 9
access-list 1 permit

The advantage of this NAT method is that all your router needs to have is one public IP Address configured on the interface facing the Internet. Ports used by communication at layer 4 will be used as distinguishing elements to allow router keep the track of the existing sessions.


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