Objective 1.7
Describe private IPv4 addressing
IPv4 has only about 4.3 billion addresses, which is fewer than the number of devices on Earth. Two mechanisms stretch it: private addressing, which lets organizations reuse the same address ranges internally, and NAT (Network Address Translation, Domain 4), which translates those private addresses to a few public ones at the Internet edge.
RFC 1918 defines three blocks of private addresses. They are never routed on the public Internet; ISPs drop packets with these sources or destinations. Any organization may use them freely inside its own network, which is why almost every home network is 192.168.0.0/24 or 192.168.1.0/24, and why enterprises typically carve up 10.0.0.0/8.
| RFC 1918 block | Address range | Number of addresses | Classful equivalent |
|---|---|---|---|
| 10.0.0.0/8 | 10.0.0.0 to 10.255.255.255 | 16,777,216 | One Class A |
| 172.16.0.0/12 | 172.16.0.0 to 172.31.255.255 | 1,048,576 | 16 Class Bs (172.16 through 172.31) |
| 192.168.0.0/16 | 192.168.0.0 to 192.168.255.255 | 65,536 | 256 Class Cs |
The 172.16.0.0/12 block is the one candidates get wrong. It covers second octets
16 through 31 only: 172.15.x.x and 172.32.x.x are public. Similarly
192.169.x.x and 11.x.x.x are public.
Any address that is not private or otherwise reserved is a public address, globally unique and assigned by regional Internet registries (ARIN, RIPE, APNIC, and others) through ISPs. A device with only a private address needs NAT to reach the Internet. Private addressing also provides a mild security benefit: hosts behind NAT are not directly reachable from outside unless the NAT device is configured to forward to them.
Other special-purpose IPv4 ranges you should recognize:
| Range | Name | Purpose |
|---|---|---|
| 127.0.0.0/8 | Loopback | Traffic to 127.0.0.1 never leaves the host; used to test the local TCP/IP stack |
| 169.254.0.0/16 | APIPA / link-local (RFC 3927) | A host that cannot reach a DHCP server assigns itself 169.254.x.x; it can talk to the local LAN only |
| 0.0.0.0/8 | “This network” | 0.0.0.0 as source before an address is known; 0.0.0.0/0 is the default route |
| 100.64.0.0/10 | Shared address space (RFC 6598) | Carrier-grade NAT inside ISP networks |
| 224.0.0.0/4 | Multicast | Class D; 224.0.0.5 and 224.0.0.6 are OSPF |
| 255.255.255.255 | Limited broadcast | To every host on the local subnet; never forwarded by routers |
| 192.0.2.0/24, 198.51.100.0/24, 203.0.113.0/24 | Documentation (RFC 5737) | Used in examples; never assigned |
APIPA (Automatic Private IP Addressing) is Microsoft’s name for the 169.254.0.0/16 behavior, but macOS and Linux do it too. If your PC shows an address starting with 169.254, it means DHCP failed: the DHCP server is down, the port is in the wrong VLAN, or the DHCP relay is missing. That is a classic exam troubleshooting clue.