Network+ (N10-009) · ELI Explains IP Addresses (book)
NAT and CIDR
On this page 3 sections
In 30 seconds
This chapter covers Network Address Translation and how IP address ranges are expressed and subdivided.
The college version
NAT (Network Address Translation)
What It Is
A router technique translating private IP addresses to a public IP and back, letting multiple devices share one internet connection.
What It Does
Rewrites outgoing source addresses to the router's public address, tracking mappings so responses return correctly.
Where It Is Used
Nearly every home router uses NAT — it's why the IPv4 internet has survived despite address exhaustion.
ELI Explains
NAT is like an apartment mailroom. Everyone inside uses apartment numbers (private), but mail goes through one street address. The mailroom delivers each envelope to the right apartment. Nobody outside knows the apartment numbers.
A 10-Year-Old's Example
Your class sends thank-you letters using the school's return address. The office sorts replies and hands each to the right student. NAT works exactly the same way.
Remember This
NAT translates private to public addresses at the boundary — it enables sharing, not security.
PAT (Port Address Translation)
What It Is
NAT using port numbers to map many private IPs to one public IP. Also called NAT overload.
What It Does
Allows hundreds of devices to share one public IP by tracking which device initiated each connection via ports.
Where It Is Used
The NAT type in virtually every home router. Your family's devices share one ISP address via PAT.
ELI Explains
PAT is like a restaurant kitchen. Many orders through one door, each with a ticket. The kitchen uses tickets to send food to the right table. PAT uses port numbers the same way.
A 10-Year-Old's Example
Your school has one phone number. Parents call, the office asks "Which student?" and transfers. Many calls, one number, tracked by who asked. PAT does this with data.
Remember This
PAT uses port numbers so many devices share one public IP — this is what home routers do.
CIDR Notation
What It Is
Classless Inter-Domain Routing: address/prefix-length (e.g., 192.168.1.0/24), where the number after the slash is network bits.
What It Does
Replaced classful addressing (Class A/B/C) with flexible prefixes, greatly improving address allocation efficiency.
Where It Is Used
Every modern network uses CIDR. 255.255.255.0 expresses the same concept — /24 is simpler and preferred.
ELI Explains
CIDR is like a zip code for a neighborhood. "Springfield/5" means "first five digits are the city, rest are streets." Compact way to say "this much is network, that much is individual houses."
A 10-Year-Old's Example
School rooms like "3B" — "3" is floor (network), "B" is room (host). /24 says "first three chunks describe which network, last chunk which device."
Remember This
CIDR (address/prefix-length) replaced classful addressing — /24 means first 24 bits are network.
Subnet Mask
What It Is
A 32-bit value (e.g., 255.255.255.0) splitting an IP address into network and host portions.
What It Does
A device applies its mask to decide whether a destination is on the same local network or needs routing.
Where It Is Used
Every IPv4 config includes a subnet mask, provided by DHCP or set manually alongside the IP and gateway.
ELI Explains
A subnet mask is like drawing a line through an address: "left of this line is street name, right is house number." Your computer uses it to separate "neighborhood" from "specific house."
A 10-Year-Old's Example
"Room 12, Green Valley Elementary." The mask highlights "Green Valley Elementary" (network) in one color and "Room 12" (your room) in another. It splits school name from room number.
Remember This
A subnet mask (255.255.255.0 = /24) splits IPs into network and host portions.
Study tools & related lessonsRelated
Educational content only. It is not medical, legal or professional advice. Found an error? Tell us.