Lecture Notes: Network Address Translation (NAT) and IPv6

Introduction

• Discussion on the limitations of IPv4 addressing due to 32-bit space.
• Origins of Network Address Translation (NAT) and IPv6 in the 1990s.
• Address exhaustion concern was a key motivator.
• NAT and IPv6 offer various advantages and are widely deployed.

Network Address Translation (NAT)

How NAT Works

• Devices within a local network use private IP addresses.
• Communication within the network uses these private addresses.
• For outside communication, NAT comes into play.
• All outgoing datagrams from devices in the network use the same 32-bit IP address.

Private IP Address Ranges

• Three private IP address ranges are used.
• Commonly seen in home, institutional, and cellular networks.

Advantages of NAT

• Single 32-bit source IP for all outgoing datagrams.
• Flexibility in changing local addresses without external notification.
• ISP changes do not require address changes within the local network.
• Security benefits: internal devices are not directly visible to the outside world.

Implementation of NAT

• Outgoing datagrams: Replace source IP and port with NAT IP and new source port.
• NAT translation table: Stores mapping of local source IP/port to NAT IP/new source port.
• Incoming datagrams: Replace destination IP and port with those from NAT table.

Example

• Demonstration with NAT address 138.76.29.7.
• Steps showing how datagrams are handled and translated.

Controversies and Challenges

• Initially controversial due to its interference with port numbers.
• NAT traversal is challenging when external hosts initiate contact.
• Despite drawbacks, NAT is widely used.

IPv6

Motivation

• Larger 128-bit address space to address IPv4 exhaustion.
• Other motivations: need for fast processing, improved flow handling.

Innovations in IPv6

• Simplified IP forwarding by eliminating variable length headers, checksums.
• Introduces flow label for flow-based services.

IPv6 Datagram Format

• 128-bit source and destination addresses.
• 16-bit flow label field for flow identification.
• 8-bit traffic class field for prioritization.

Fields Removed in IPv6

• No checksum, fragmentation/reassembly, or options fields.
• Fixed length header for faster processing.

Transition from IPv4 to IPv6

• Coexistence of both protocols during transition.
• Use of tunneling to allow interoperation.

Tunneling Technique

• IPv6 datagram encapsulated within an IPv4 datagram.
• Used to connect IPv6 routers over an IPv4 network.

Example of Tunneling

• Description of the process with routers supporting IPv6 and IPv4.
• Demonstration of datagram forwarding through a mixed network.

Current Deployment of IPv6

• 30% of Google’s clients use IPv6.
• One-third of U.S. government domains support IPv6.
• NAT deployment reduces IPv6 adoption urgency.

Application Layer Innovation vs. Network Layer Changes

• Rapid application layer changes contrast with slower network layer shifts.

Conclusion

• Covered IPv4, NAT, and IPv6 in-depth.
• Set the stage for generalized forwarding and software-defined networking discussions.