Public, Private, and Loopback Addresses

IPv4 addresses fall into two broad groups: public addresses that are routable on the global internet, and private addresses reserved for use inside local networks.

This division lets organizations reuse the same private ranges internally while sharing a small pool of public addresses for internet access.

A public IP address is unique across the entire internet and is assigned by an ISP (Internet Service Provider) or a regional registry. Web servers, mail servers, and your home router’s internet side use public addresses.

Because public addresses are globally routable, any device on the internet can reach them directly, which is why they are a limited and managed resource.

The standard RFC 1918 defines three private ranges that are never routed on the public internet:

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

Home and office LANs use these freely without coordination.

Millions of homes use 192.168.1.0 internally at the same time. This causes no conflict because private addresses stay inside each local network and are never seen on the internet.

The reuse is possible because NAT (Network Address Translation) converts private addresses to a public one at the router boundary, a topic covered in a later course.

The loopback range 127.0.0.0/8 lets a device talk to itself. The most-used address is 127.0.0.1, often named "localhost".

Traffic to a loopback address never leaves the device; it is handled internally by the network stack. This makes loopback ideal for testing software and confirming TCP/IP is installed correctly.

When a device cannot reach a DHCP server, it may assign itself an APIPA address (Automatic Private IP Addressing) from 169.254.0.0/16.

An address starting with 169.254 is a strong sign that DHCP failed. APIPA allows limited local communication but provides no gateway, so the device cannot reach the internet.

You can classify an address at a glance:

• Starts with 10, 172.16-31, or 192.168 → private
• Starts with 127 → loopback
• Starts with 169.254 → APIPA (DHCP failure)
• Anything else routable → likely public

This quick triage speeds up troubleshooting enormously.

NAT is the glue between the private and public worlds. Your router holds a single public address from the ISP and translates the private addresses of all your devices to that one public address for internet traffic.

NAT is the main reason IPv4 survived long past its theoretical exhaustion, letting many devices share one public address.

Open your home device settings and you will likely see a private address such as 192.168.1.23. Search "what is my IP" in a browser and you will see your router’s public address instead.

The gap between those two numbers is NAT at work, hiding your internal layout behind one internet-facing address.

Keeping devices on private addresses provides a basic layer of protection. Outside hosts cannot directly initiate connections to a private address, since it is unroutable on the internet.

This is not a substitute for a firewall, but it does mean internal devices are not directly exposed simply by being online.

Memorize these special ranges for the exam:

• Private: 10/8, 172.16/12, 192.168/16
• Loopback: 127.0.0.0/8
• APIPA: 169.254.0.0/16

Recognizing them instantly tells you whether an address is internal, a self-test, or a sign of DHCP trouble.

Diagnose this address.

You classified IPv4 address types.

• Public addresses are globally routable and ISP-assigned.
• Private (RFC 1918): 10/8, 172.16/12, 192.168/16 — local only.
• Loopback 127.0.0.1 tests the local stack.
• APIPA 169.254.x.x signals DHCP failure.
• NAT lets many private devices share one public address.