Private and Public IP Addressing

Every packet that traverses the public Internet carries two critical pieces of information: a source address and a destination address. For global communication to function, these addresses must be globally unique—no two devices anywhere on Earth can legitimately claim the same public IP address at the same time. This seemingly simple requirement has spawned one of the most complex administrative systems in computing: the global IP address allocation hierarchy.

Public addresses are the finite, carefully managed resource that makes the Internet possible. Unlike private addresses that can be reused across millions of independent networks, each public IP address represents a specific, unique point of presence on the global network. The total supply—approximately 3.7 billion usable addresses—must serve an interconnected world of over 8 billion people and tens of billions of devices.

The distribution of IPv4 addresses follows a carefully structured hierarchy, designed to ensure fair, documented, and technically sound allocation across the entire Internet. At the apex of this hierarchy sits the Internet Assigned Numbers Authority (IANA), which manages the global pool of unallocated addresses and delegates large blocks to regional authorities.

The three-tier hierarchy:

                    ┌─────────────┐
                    │    IANA     │  (Global Authority)
                    └──────┬──────┘
                           │
     ┌─────────┬─────────┬─┴───┬─────────┬─────────┐
     │         │         │     │         │         │
 ┌───┴───┐ ┌───┴───┐ ┌───┴───┐ ┌───┴───┐ ┌───┴───┐
 │ ARIN  │ │ RIPE  │ │ APNIC │ │LACNIC │ │AFRINIC│  (Regional)
 └───┬───┘ └───┬───┘ └───┬───┘ └───┬───┘ └───┬───┘
     │         │         │         │         │
   ┌─┴─┐     ┌─┴─┐     ┌─┴─┐     ┌─┴─┐     ┌─┴─┐
   │NIR│     │LIR│     │NIR│     │LIR│     │LIR│    (National/Local)
   └─┬─┘     └─┬─┘     └─┬─┘     └─┬─┘     └─┬─┘
     │         │         │         │         │
   ┌─┴─┐     ┌─┴─┐     ┌─┴─┐     ┌─┴─┐     ┌─┴─┐
   │End│     │End│     │End│     │End│     │End│    (End Users)
   │Usr│     │Usr│     │Usr│     │Usr│     │Usr│
   └───┘     └───┘     └───┘     └───┘     └───┘

This hierarchical structure ensures that address allocation decisions are made by organizations with regional expertise while maintaining global coordination and preventing conflicts.

Regional Internet Registries are the operational heart of address allocation. Each RIR manages IP address resources for a specific geographic region, operating as member-based organizations governed by the Internet community within their service areas. The five RIRs collectively serve the entire world.

RIR responsibilities include:

1. Address allocation — Receiving large blocks from IANA and allocating them to Local Internet Registries (LIRs), typically ISPs or large organizations
2. Policy development — Facilitating community-driven policy processes that determine allocation rules
3. Registry maintenance — Operating the authoritative databases (WHOIS/RDAP) documenting who holds which addresses
4. Reverse DNS delegation — Managing the in-addr.arpa zones for reverse DNS lookups
5. Community coordination — Hosting meetings, training, and technical assistance programs
6. Resource certification — Providing RPKI (Resource Public Key Infrastructure) services for route origin validation

The most significant fact about public IPv4 addresses today is their scarcity. IPv4 exhaustion—the depletion of the global pool of unallocated addresses—is not a future concern; it is a present reality that has fundamentally transformed how organizations acquire and manage IP addresses.

The exhaustion timeline:

Post-exhaustion operations:

RIRs continue to operate after exhaustion, but their allocation models have changed dramatically:

• Waiting lists — Organizations in ARIN's region join multi-year waiting lists for addresses that become available through returns
• Minimal allocations — RIPE NCC allocates only /24 blocks (256 addresses) to new members, regardless of demonstrated need
• Transfer markets — The primary mechanism for obtaining addresses is now purchasing them from existing holders

Address reclamation efforts:

Before complete exhaustion, significant efforts recovered underutilized addresses:

Despite these efforts, exhaustion was inevitable given Internet growth rates. The 4.3 billion address limit cannot accommodate a world where every smartphone, laptop, server, IoT device, and vehicle demands connectivity.

Public addresses fall into two fundamental categories based on their relationship to Internet service providers. Understanding this distinction is crucial for network architects making decisions about address acquisition and multi-homing strategies.

Provider-Aggregatable (PA) addresses:

PA addresses are allocated to end users from their ISP's address block. The addresses 'belong' to the ISP and are merely delegated to the customer for the duration of the service relationship.

Provider-Independent (PI) addresses:

PI addresses are assigned directly to an organization by an RIR, independent of any particular ISP. The organization maintains these addresses regardless of their upstream connectivity.

PA Addressing:                        PI Addressing:

┌─────────────────┐                  ┌─────────────────┐
│   ISP Block     │                  │   RIR Registry  │
│  203.0.113.0/24 │                  │                 │
└────────┬────────┘                  └────────┬────────┘
         │                                    │
         │ Delegation                         │ Direct Assignment
         ▼                                    ▼
┌─────────────────┐                  ┌─────────────────┐
│   Customer      │                  │   Organization  │
│ 203.0.113.64/26 │                  │  198.51.100.0/24│
└─────────────────┘                  └─────────┬───────┘
         │                                    │
    ISP Change                           ISP Change
    = Renumber!                          = Just BGP!
                                              │
                                    ┌─────────┼─────────┐
                                    ▼         ▼         ▼
                                  ISP A     ISP B     ISP C

Public IPv4 addresses are organized into blocks defined by CIDR notation. Understanding how these blocks are structured, sized, and allocated is fundamental to network planning and Internet routing comprehension.

/8 blocks (legacy Class A):

/8 blocks represent the largest units in IPv4 addressing, containing over 16 million addresses each. Originally assigned to single organizations, these massive allocations were made before anyone anticipated how valuable addresses would become.

Historic /8 allocations include:

Modern allocation sizes:

Today's allocations are much smaller, reflecting scarcity and fair distribution policies:

The /24 minimum:

The Internet routing system has practical limits. Most ISPs filter announcements smaller than /24, meaning a /25 (128 addresses) or smaller cannot be independently routed. This creates the effective minimum block size for PI addressing:

/24 = 256 addresses (minimum routable)
/25 = 128 addresses (typically filtered, not routable independently)
/26 = 64 addresses (definitely filtered)

In the post-exhaustion era, organizations have several pathways to obtain public IPv4 addresses, each with distinct costs, timelines, and considerations.

Not all publicly routable address space is available for general assignment. Certain blocks are reserved for special purposes, and understanding these reservations prevents confusion and configuration errors.

Well-known public addresses:

Certain public addresses have become widely recognized due to their widespread use in public services:

These addresses are sometimes hardcoded in applications and network configurations, creating implicit dependencies on their continued operation.

The ability to determine who operates any given public IP address is fundamental to Internet operations, security incident response, and abuse mitigation. WHOIS databases—and their modern successor RDAP—provide this crucial lookup capability.

Querying WHOIS:

# Command-line WHOIS query
$ whois 8.8.8.8

NetRange:       8.0.0.0 - 8.255.255.255
CIDR:           8.0.0.0/8
NetName:        LVLT-ORG-8-8
NetHandle:      NET-8-0-0-0-1
Parent:         ()
NetType:        Direct Allocation
Organization:   Google LLC (GOGL)
RegDate:        2023-12-28
Updated:        2023-12-28
Ref:            https://rdap.arin.net/registry/ip/8.0.0.0

OrgName:        Google LLC
OrgId:          GOGL
Address:        1600 Amphitheatre Parkway
City:           Mountain View
StateProv:      CA
PostalCode:     94043
Country:        US
...

RDAP (Registration Data Access Protocol):

RDAP is the modern, structured replacement for WHOIS, offering:

• JSON-formatted responses for programmatic access
• HTTPS transport with authentication support
• Standardized output across all RIRs
• Better internationalization support

Public IP addresses are the globally unique identifiers that enable Internet communication. Their scarcity, careful management, and economic value make understanding them essential for any network professional.

What's next:

Having explored both private and public addressing independently, the next page examines address conservation—the strategies and technologies developed to extend IPv4's useful life despite exhaustion. Understanding conservation illuminates why NAT became ubiquitous and why IPv6 adoption remains slow despite pressing need.