Reserved IP Address Blocks Explained (Private, CGNAT, Loopback, and More)

Not every IP address on the internet is "public." Large ranges are reserved for private networks, local communication, documentation, testing, and special routing behavior. Knowing these reserved IP blocks helps you troubleshoot networks and avoid confusing an internal address with a public one.

Private IP ranges (RFC1918)

Private IPs are used inside home and office networks. They are not routable on the public internet.

• 10.0.0.0/8 (10.0.0.0 - 10.255.255.255)
• 172.16.0.0/12 (172.16.0.0 - 172.31.255.255)
• 192.168.0.0/16 (192.168.0.0 - 192.168.255.255)

If you want a clear explanation of why these exist, read Public vs Private IP addresses.

Carrier-Grade NAT (CGNAT): 100.64.0.0/10

Many ISPs use CGNAT to share a smaller pool of public IPv4 addresses across many customers. If you see an address in 100.64.0.0 - 100.127.255.255, it's not a normal "public IP" you control - it's typically an ISP-side internal range.

Loopback: 127.0.0.0/8

127.0.0.1 is the classic "localhost." Loopback traffic never leaves your device; it's used for testing services locally.

Link-local: 169.254.0.0/16

Link-local addresses often appear when a device can't get an IP from DHCP. It's a sign something's wrong with the network, not that the device is online with a public IP.

Documentation ranges (examples in guides)

These are reserved so tutorials can use "fake" IPs that won't accidentally point to real systems:

• 192.0.2.0/24 (TEST-NET-1)
• 198.51.100.0/24 (TEST-NET-2)
• 203.0.113.0/24 (TEST-NET-3)

Multicast: 224.0.0.0/4

Multicast is used for one-to-many communication (like some discovery protocols). These addresses are not normal device addresses.

So... what's my real public IP?

Your public IP is what websites see when you connect. If you're on a home network, your devices usually have private IPs, and your router has (or shares) the public IP.

You can check what your current connection exposes using our IP Address Lookup. If you're learning CIDR ranges (like /24, /16), see CIDR notation explained.

Conclusion

Reserved IP blocks exist so networks can function safely and predictably - private routing, local testing, documentation, and ISP address sharing all depend on them. Once you recognize the ranges, troubleshooting gets a lot faster.