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NAT and IP Addressing

Sep 2, 2025

Overview

This lecture explains how Network Address Translation (NAT) enables billions of devices to share a limited number of IPv4 addresses when accessing the internet.

IPv4 Address Limitations

  • IPv4 supports about 4.29 billion unique addresses.
  • The available IPv4 address pool has been completely exhausted.
  • Billions of new internet-connected devices need solutions to overcome this limitation.

Private vs. Public IP Addresses

  • Private IP addresses are not routable over the public internet and are defined by RFC 1918.
  • Common private address ranges: 10.0.0.0 - 10.255.255.255, 172.16.0.0 - 172.31.255.255, 192.168.0.0 - 192.168.255.255.
  • Private IPs are used within homes and businesses; public IPs are needed to access internet resources.

Network Address Translation (NAT)

  • NAT allows private IP addresses to communicate with the internet by translating them into public IPs.
  • A NAT-enabled router replaces the private source IP address with a public IP address for outbound internet traffic.
  • The router keeps a translation table to map public IPs back to the original internal private IPs during inbound responses.

Port Address Translation (PAT) / NAT Overload

  • PAT (also called NAT overload) maps multiple private IP addresses to a single public IP using different port numbers.
  • Each outgoing connection from a private address is assigned a unique source port on the public IP.
  • This allows many users/devices to share a single public IP while maintaining unique sessions.

Key Terms & Definitions

  • IPv4 Address — A 32-bit unique identifier for devices on a network.
  • Private IP Address — An IP address not routable on the public internet, used within private networks.
  • Public IP Address — An IP address that is routable on the public internet.
  • NAT (Network Address Translation) — A process of modifying IP address information in IP packet headers while in transit.
  • PAT (Port Address Translation) — A type of NAT where multiple devices share a single public IP by using different port numbers (also called NAT overload).

Action Items / Next Steps

  • Review RFC 1918 for details on private IP address ranges.
  • Understand how NAT and PAT tables function on network routers.

Full transcript