NETWORK
Public vs private IP addressesRFC 1918 ranges and what they mean
A private IP address comes from the ranges reserved in RFC 1918 (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) and is only meaningful inside a local network; millions of networks reuse the same private addresses. A public IP address is globally unique and routable on the internet. NAT on the router translates between the two.
The two kinds of address
IPv4 has about 4.3 billion addresses, far fewer than the number of connected devices. The solution adopted in the 1990s was to give each network a small number of globally unique public addresses and let devices inside the network use private addresses that are never routed on the internet. A router performing NAT rewrites private source addresses to its public address on the way out and reverses the mapping on the way back.
| Property | Private | Public |
|---|---|---|
| Unique? | Only within one network | Globally |
| Routable on the internet? | No; ISPs drop it | Yes |
| Who assigns it? | Your router or administrator | Your ISP or a regional registry |
| Reachable from outside? | Only via port forwarding or VPN | Directly |
The private ranges
| Range | CIDR | Addresses | Typical use |
|---|---|---|---|
| 10.0.0.0 – 10.255.255.255 | 10.0.0.0/8 | 16.7 million | Enterprises, cloud VPCs |
| 172.16.0.0 – 172.31.255.255 | 172.16.0.0/12 | 1 million | Docker default networks, mid-size organisations |
| 192.168.0.0 – 192.168.255.255 | 192.168.0.0/16 | 65,536 | Home and small-office routers |
Other special ranges that look private
- 100.64.0.0/10 — shared address space for carrier-grade NAT. If your router's WAN address is in this range, your ISP is applying NAT a second time, and inbound port forwarding will not work.
- 169.254.0.0/16 — link-local. Windows assigns an address here (APIPA) when DHCP fails; seeing
169.254.x.xinipconfigmeans "no DHCP server answered". - 127.0.0.0/8 — loopback; the machine itself.
- 192.0.2.0/24, 198.51.100.0/24, 203.0.113.0/24 — reserved for documentation, never assigned. Examples on this site use them.
IPv6
IPv6 has enough addresses that every device can have a public one, so NAT is not needed. Two private-style ranges still exist: fe80::/10 link-local addresses that every interface has automatically, and fc00::/7 unique local addresses for networks that want internal-only addressing. See IPv4 vs IPv6.
Finding out what you have
ipconfigIPv4 Address under your adapter is almost always private. The router's admin page shows the WAN address. Compare the two: if they differ and the WAN address is public, you are behind one layer of NAT, which is the normal home situation.
Why it matters
- Hosting anything (a game server, a camera, a web service) requires a public address somewhere and a forwarding rule, or a tunnel that avoids inbound connections.
- Security: private addresses are not reachable from the internet by default, which is a useful side effect but not a security control on its own; malware that phones home still works fine.
- VPN design: two sites that both use 192.168.1.0/24 cannot be joined without renumbering or NAT, a frequent problem with home-office VPNs.
Frequently asked questions
Is 192.168.1.1 a private IP?
Yes. It is inside 192.168.0.0/16 and is the most common default address for home routers. It exists in millions of homes at once, which is exactly what private space is for.
How do I find my public IP address?
Your devices only know their private addresses. The router's status page shows its WAN address, and any "what is my IP" website reports the public address your traffic arrives from. If the router's WAN address is itself private (especially 100.64.x.x), your ISP uses carrier-grade NAT.
Can two devices have the same private IP?
Not on the same network; that causes an address conflict. On different networks it is normal and harmless because the addresses never meet without NAT in between.
Is 172.16.x.x always private?
Only 172.16.0.0 through 172.31.255.255 (the /12). 172.32.0.0 onward is public space.