NAT - PAT - Overload

Network Address Translation

• Network address translation (NAT)

• Translate from one address to another

• Commonly used for private to another

• Commonly used for private IP > public IP

• Extends the life of IPv4

• Common NAT types:

  • Static NAT
    • One-to-one translation.
  • Dynamic NAT
    • Private IP address translates to one of a pool of public IP address.
  • Port Address Translation (PAT)
    • Many-to-one
    • A single IP address on the outside.
      • Can save money and helps scalability

• There is such thing as double NATing

  • Very very hard to troubleshoot.

• "NAT is what allows us to have ~4 billion network devices working on the internet together."

• One-to-one
• We can take the private IP and convert it into a public IP
• Inbound traffic is denied by default
• Outbound traffic is allowed by default

Static NAT

• 1:1 mapping of addresses
• Set by administrator
• Referred to as "one-to-one"

Configuration example:

ip nat inside source static 10.1.1.5 23.5.5.9
ip nat inside source static 10.1.1.10 23.5.5.10
ip nat inside source static 10.1.1.15 23.5.5.11

• Take an internal IP and map it to an external IP
• Create DNS records
  • A records
    • A = wwww IP
    • A = mail IP

Port Address Translation

• "The reason why NAT exists and conserves the public address space"

• Port numbers assits with translation

• Many users share one public address by overloading the interface

• Most common form of NAT

• For very large networks we usually combine Dynamic NAT with PAT

  • Usually done with a simple overload command.

• Starts with an Access Control List (ACL)

  • An if-then condition

Configuration example:

access-list 1 10.1.1.0 0.0.0.255
ip nat inside source list 1 interface fa/01 overload

• The router can use its own interface for the PAT traffic
• Note the overload operation.
• We use the port number to keep each session unique.

NAT Table

• Inside Global - NAT Address (could be public or private)
• Inside Local - Host's Actual Address
• Outside Global - Destination Address
• Outside Local - Destination Address

Router#show ip nat translations