Public vs Private Addresses
Every laptop at Kite Studio has an address starting 192.168.10. So does the laptop of a designer in the office next door — maybe the very same 192.168.10.57. Nothing clashes, because those addresses never appear on the internet. Only the studio router's 203.0.113.10 does. Telling the two kinds apart at a glance is a daily skill.
The rule
Private addresses stay inside; only public addresses travel the internet.
Private addresses are three IPv4 ranges set aside for use inside any organisation, free and without asking anyone. Internet routers refuse to carry them, so the same private address can be reused in millions of buildings:
| Range | CIDR | Size |
|---|---|---|
10.0.0.0 – 10.255.255.255 |
10.0.0.0/8 |
about 16.7 million addresses |
172.16.0.0 – 172.31.255.255 |
172.16.0.0/12 |
about 1 million addresses |
192.168.0.0 – 192.168.255.255 |
192.168.0.0/16 |
65,536 addresses |
Public addresses are unique across the whole internet. They are allocated through regional registries to ISPs and organisations, and they are what internet routers carry. Kite Studio rents exactly one, 203.0.113.10, from its ISP. (The 203.0.113.x and 198.51.100.x blocks are reserved for documentation, so this course uses them to stand in for real public addresses.)
Other special ranges you will meet:
127.0.0.0/8— loopback;127.0.0.1means "this machine itself".169.254.0.0/16— link-local, self-assigned when DHCP fails.100.64.0.0/10— shared space some ISPs use for their own large-scale NAT. It is neither your private LAN nor a public address.
In IPv6 the idea is similar but less needed: most IPv6 addresses are globally unique, and fc00::/7 (in practice fd00::/8) plays the private role.
The shortcut
Check the first octet, then only if needed the second:
10→ private, always.192→ private only if the second octet is168.172→ private only if the second octet is 16 to 31.- anything else → public (unless it is one of the special ranges above).
10.4.2.9First octet 10: private.
The traps are the edges of the 172 range, and assuming every 192.x address is private. Only 192.168.x.x is.
Drills
Answer fast — trust the rule.
Which address is private?
Which address could appear as a source on the public internet?
Why can two neighbouring offices both use 192.168.10.57 without a clash?
A developer runs a server that listens on 127.0.0.1. Who can reach it?
Remember
- Private addresses stay inside; only public addresses travel the internet.
- Private: 10/8, 172.16–172.31, 192.168/16.
- Watch the edges: 172.15 and 172.32 are public, and only 192.168 in the 192 range is private.