Lesson 39: Reserved Addresses (Private, APIPA, Loopback)
Not all IP addresses are meant for use on the public internet. Several blocks are reserved for special purposes.
1. Private IP Address Space
These address blocks are reserved for use within private networks (LANs). They are non-routable, meaning routers on the public internet will discard any packet containing a private source or destination IP address.
| Class | Reserved Range | CIDR Notation |
|---|---|---|
| Class A | 10.0.0.0 to 10.255.255.255 | 10.0.0.0/8 |
| Class B | 172.16.0.0 to 172.31.255.255 | 172.16.0.0/12 |
| Class C | 192.168.0.0 to 192.168.255.255 | 192.168.0.0/16 |
Key Use: Your home Wi-Fi network uses 192.168.1.x, which is a common private address.
2. Loopback Address
- Address:
127.0.0.1(or the entire127.0.0.0/8range). - Purpose: Used for local testing. Traffic sent to the loopback address never leaves the host machine. It is used to test if the TCP/IP stack on the host is functioning correctly.
3. Automatic Private IP Addressing (APIPA)
- Address Range:
169.254.0.0to169.254.255.255(169.254.0.0/16). - Purpose: If a device is configured to get an IP address automatically (via DHCP) but cannot find a DHCP server, it will assign itself an APIPA address. APIPA allows devices on the same segment to still communicate locally, but they cannot access the internet.
Troubleshooting Tip: If your device has a 169.254.x.x address, your DHCP server or network connection is likely broken.