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Private Addressing Refresher

RFC 1918 defines three private IP address ranges that are not routable on the public internet, allowing organizations to use Network Address Translation (NAT) to map many internal devices to one or more public IP addresses. This approach emerged from decades of IPv4 exhaustion workarounds, from classful addressing and subnetting to CIDR, before IPv6 ultimately provided a long-term solution.

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About this video

The history of IP addressing is largely a story of running out of space and engineering around the constraint. When IPv4 was first defined in 1980 under RFC 760, the entire address space was divided by only the first eight bits, yielding just 256 possible networks. RFC 791 introduced classful addressing in 1981, segmenting the space into Class A, B, and C blocks to dramatically increase the number of available networks. When that proved insufficient, RFC 950 introduced subnetting in 1985, allowing large address blocks to be divided into smaller, more manageable segments. CIDR and variable-length subnet masking (VLSM) followed, enabling subnets of varying sizes within the same address space and making allocation far more efficient. By 1996, those measures still could not keep pace with demand, and RFC 1918 established a new approach by designating three address ranges as private and explicitly non-routable on the public internet: 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. These ranges were drawn from Class A, B, and C space respectively, giving network administrators flexibility to support everything from small home networks to large enterprise environments. Because these addresses are unroutable externally, they can be reused freely across any number of independent private networks without conflict. Network Address Translation (NAT) is the mechanism that makes private addressing practical for internet-connected environments. When traffic from a privately addressed device needs to reach the public internet, a router translates the source address to a publicly routable IP before forwarding the packet. Return traffic arrives at that public address and is then translated back and delivered to the correct internal device. This allows a single public IP address to represent an entire network of private addresses, multiplying the effective capacity of the IPv4 address space considerably. IPv6, standardized under RFC 2460, ultimately addressed exhaustion at the root by expanding from 32-bit to 128-bit addresses, but RFC 1918 private addressing combined with NAT remains foundational to how most networks operate today.

What you'll learn

What's covered

Private Addressing & NAT

Aligned to

Cisco CCNA
1.7 Describe private IPv4 addressing
4.1 Configure and verify inside source NAT using static and pools

Key terms

IP Address
A numerical label assigned to each device connected to a network that uses the Internet Protocol.
Network Address Translation
NAT
A method that maps private IP addresses to a public IP address, allowing multiple devices to share a single public IP.
Subnet
A logical subdivision of an IP network, used to organize and segment network traffic.
RFC 1918
The internet standard that defines three private IPv4 address ranges reserved for use within private networks and not routable on the public internet.
Private IP Address
An IP address from a reserved range (Class A: 10.0.0.0–10.255.255.255, Class B: 172.16.0.0–172.31.255.255, Class C: 192.168.0.0–192.168.255.255) used within private networks and not routable on the public internet.
Public IP Address
A globally unique IP address assigned to a device or network that is routable and reachable across the public internet.

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