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Module - 6 Network Address Translation For IPv4

2026-10-04 13:43

Tags: #enterpriseNetwork

Author: Duke Hsu

Topic

  1. NAT definition
  2. NAT Terminology
  3. Types of NAT
  4. NAT Advantages and Disadvantages
  5. NAT for IPv4 Hands-on
  6. NAT 64

1. NAT Definition

NAT allows networks to use private IPv4 addresses internally and translates them to a public address when needed

A NAT router typically operates at the border of a stub network

When a device inside the stub network wants to communicate with a device outside of its network, the packet is forwarded to the border router which performs the NAT process, translating the internal private address of the device to a public , outside, routable address.

1.1 How NAT Works

How-NAT-Works

PC1 wants to communicate with an outside web server with
public address 209.165.201.1.

  1. PC1 sends a packet addressed to the web server.
  2. R2 receives the packet and reads the source IPv4 address
    to determine if it needs translation.
  3. R2 adds mapping of the local to global address to the NAT
    table.
  4. R2 sends the packet with the translated source address
    toward the destination.
  5. The web server responds with a packet addressed to the
    inside global address of PC1 (209.165.200.226).
  6. R2 receives the packet with destination address
    209.165.200.226. R2 checks the NAT table and finds an
    entry for this mapping. R2 uses this information and
    translates the inside global address (209.165.200.226) to
    the inside local address (192.168.10.201), and the packet is
    forwarded toward PC1

2. NAT Terminology

NAT includes four types of addresses:

  • Inside local address
  • Inside Global address
  • Outside local address
  • Outside global address

image.png

3. Types of NAT

3.1 Static NAT

Static NAT uses a one-to-one mapping of local and global addresses configured by the network administrator that remain constant.

  • Static NAT is useful for web server or devices that must have a consistent address that is accessible from the internet, such as a company web server.

  • It is also useful for devices that must be accessible by authorized personnel when offsite, but not by the general public on the internet.

Information

Static NAT requires that enough public addresses are available to satisfy the total number of simultaneous user sessions.

Static NAT configuration

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R2(config)#interface gigabitEthernet 0/1 #inside globe 
    R2(config-if)#ip address 209.165.200.226 255.255.255.252
    R2(config-if)#ip nat outside
    R2(config-if)#no shutdown
    R2(config-if)#exit
R2(config)#interface gigabitEthernet 0/0 #inside local
    R2(config-if)#ip address 10.0.0.2 255.255.255.252 
    R2(config-if)#ip nat inside
    R2(config-if)#no shutdown
R2(config)#ip route 0.0.0.0 0.0.0.0 209.165.200.225 #next hop
R2(config)#ip nat inside source static 192.168.10.201 209.165.200.226

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ip nat inside source static 192.168.10.10 209.165.200.226
        │      │      │         │              │
        │      │      │         │              └ Inside Global
        │      │      │         └ Inside Local
        │      │      └ Static (1 by 1)
        │      └ Source 
        └ For inside host

References