Network+ (N10-009) · ELI Explains IP Addresses (book)
Public and Private Addresses
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In 30 seconds
This chapter covers the distinction between globally routable and locally scoped IP addresses.
The college version
Public IP Address
What It Is
An IP address globally unique and routable on the public internet, assigned by an ISP or regional internet registry.
What It Does
Allows a device to be directly reachable from anywhere — essential for web servers, email, and cloud services.
Where It Is Used
Your router's internet-facing port, web servers, and cloud VMs all use public addresses.
ELI Explains
A public IP is like your home's street address — anyone can send you mail using it. When you visit a website, you connect to its public address, like driving to a street address.
A 10-Year-Old's Example
Your school's entrance has a street address anyone can find. Classrooms inside have room numbers only meaningful once you're inside. The street is public; rooms are private.
Remember This
A public IP address is globally unique and routable — it's how the internet finds you.
Private IP Address
What It Is
An IP address reserved for local networks, never routed on the public internet. Defined in RFC 1918 (IPv4) and RFC 4193 (IPv6).
What It Does
Lets devices on a home or office network communicate locally and share one public connection through NAT.
Where It Is Used
Every home Wi-Fi and office LAN uses private addresses like 192.168.x.x, 10.x.x.x, or 172.16.x.x.
ELI Explains
Your house has room numbers only meaningful inside. Your neighbor can have the same numbers — not confusing because they only matter inside each house. Private IPs work the same way.
A 10-Year-Old's Example
Your bedroom is "Room 2" and your friend's is also "Room 2" in their house. Fine — each house keeps numbers private. The mail carrier only cares about your street address.
Remember This
Private addresses need NAT to reach the internet — not directly reachable from outside.
Private IPv4 Ranges
What It Is
Three reserved RFC 1918 blocks: 10.0.0.0/8 (16M+ addresses), 172.16.0.0/12 (~1M), and 192.168.0.0/16 (65K+).
What It Does
Provides organizations of any size with private address space — from homes using 192.168.x.x to enterprises using 10.x.x.x.
Where It Is Used
Home routers default to 192.168.0.x. Large businesses and universities use 10.x.x.x for thousands of devices.
ELI Explains
Your town has three street-name sets: "Short Street" for small neighborhoods, "Medium Avenue" for bigger ones, "Long Boulevard" for the biggest. Each has its own rules. Private IP ranges work the same — pick the size that fits.
A 10-Year-Old's Example
Three devices at home use the small range, like a three-bedroom house. A company with thousands of computers uses the largest range — like a giant apartment building where every room needs a number.
Remember This
RFC 1918: 10/8 (big), 172.16/12 (medium), 192.168/16 (small) — memorize for exams.
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