Network Address Translation

Network Address Translation or NAT is a brilliant scheme to preserve IP addresses. Organizations are often assigned a limited number of public IP addresses. It would be extremely expensive to convert all private IP addresses to public addresses for outside communication. NAT offers a brilliant solution to this problem. NAT allows 3 different ways of mapping internal IP addresses to public IP addresses.

Before we dive into the different types of NATs lets first clarify the terminologies that are often confusing.

The IP address of the private network host is called inside local and once that private IP address is mapped to a public IP address it becomes inside global. For e.g in the above fig we can see that the IP address of the inside host changes for to The IP address is refereed as Inside Local and the IP address after mapping is refereed as Inside Global

And similarly, a public IP address is called outside global and when it is mapped to a private IP address it is called outside local. For e.g in the above fig we can see that the IP address of the Public host changes for to The IP address is refereed as Outside Global and the IP address after mapping is refereed as Outside Local

Now, the 3 types of NATs are

  • Static NAT
  • Dynamic NAT
  • Port Address Translation

The following lab is an example of all three nats

 Router#sh run
 Building configuration…
 Current configuration : 3655 bytes
 ! Last configuration change at 20:23:44 UTC Mon Dec 16 2019
 version 15.6
 service timestamps debug datetime msec
 service timestamps log datetime msec
 no service password-encryption
 hostname Router
 no aaa new-model
 ethernet lmi ce
 no process cpu autoprofile hog
 mmi polling-interval 60
 no mmi auto-configure
 no mmi pvc
 mmi snmp-timeout 180
 ip icmp rate-limit unreachable
 no ip domain lookup
 ip cef
 no ipv6 cef
 multilink bundle-name authenticated
 no cdp log mismatch duplex
 no cdp run
 ip tcp synwait-time 5
 interface GigabitEthernet0/0
  ip address
  ip nat outside
  ip virtual-reassembly in
  duplex auto
  speed auto
  media-type rj45
  no cdp enable
 interface GigabitEthernet0/1
  description Static NAT
  ip address
  ip nat inside
  ip virtual-reassembly in
  duplex auto
  speed auto
  media-type rj45
  no cdp enable
 interface GigabitEthernet0/2
  description Dynamic NAT
  ip address
  duplex auto
  speed auto
  media-type rj45
  no cdp enable
 interface GigabitEthernet0/3
  description PAT
  ip address
  duplex auto
  speed auto
  media-type rj45
  no cdp enable
 ip forward-protocol nd
 no ip http server
 no ip http secure-server
 ip nat pool dynamicNAT netmask
 ip nat inside source list 7 pool dynamicNAT
 !to perform PAT we would do  ip nat inside source list 7 pool dynamicNAT overload
 ip nat inside source static
 access-list 7 permit


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