IPv4 Classes, Private/Public, APIPA, and Loopback

IPv4 Addressing on the N10-009 Exam

IPv4 addressing is foundational for CompTIA Network+ N10-009, the current edition launched in 2024. The exam delivers up to 90 questions in 90 minutes, mixes multiple-choice with performance-based questions (PBQs), and requires a scaled score of 720 on the 100-900 scale to pass (not a fixed number of items correct). The exam fee in the United States is about $369 USD. Subnetting and address recognition appear across multiple domains, so you must read a leading octet or a self-assigned address and instantly name its type.

N10-009 uses five official domains with these weights. Address topics live mostly in Networking Concepts but resurface heavily in Troubleshooting.

An IPv4 address is a 32-bit number written as four 8-bit octets, each 0-255, separated by dots. The subnet mask marks which leading bits are the network portion and which trailing bits identify the host. Two reserved addresses exist in every subnet: the network ID (all host bits 0, the first address) and the broadcast address (all host bits 1, the last address). Neither is assignable to a host.

Legacy Classes

Classful addressing predates CIDR but the first-octet ranges still appear in questions. Recognize the range, then ignore the default mask unless the question is explicitly about classes.

Note the gap: 127.x.x.x is reserved for loopback, so the Class A host range stops at 126. The address 0.0.0.0 is special and means "this host," "any address," or the default route depending on context, never a normal host.

Private, Public, APIPA, and Loopback

The most common trap is the 172.16.0.0/12 boundary: 172.20.5.6 is private but 172.40.5.6 is public, because the private block ends at 172.31. The exam loves testing whether you stop counting at 31.

Troubleshooting Clues

A host's address often diagnoses the fault before you run a single command.

PBQ-Style Thinking

Scenario: a workstation shows 169.254.77.19 / 255.255.0.0 and cannot reach file shares or the internet. APIPA is the symptom, not the cause. Check DHCP server availability, the IP-helper/relay configuration on the router, the VLAN assignment on the switch port, link status, and wireless association. Restoring DHCP fixes the address.

Scenario: a printer at 192.168.10.50/24 has gateway 192.168.11.1. It talks to local hosts but cannot leave the subnet, because the gateway is not inside 192.168.10.0/24. Correct the gateway so it sits on the local network.

Why the Mask Matters More Than the Class

A persistent exam trap is reading the first octet and assuming a default mask. Consider 10.10.10.10. By classful logic it is a Class A address with a /8 mask, yet in a real network it might be configured as 10.10.10.10/24 or 10.10.10.10/30. The class tells you nothing reliable about the subnet boundary; only the mask or prefix does. CompTIA writes distractors precisely to punish candidates who answer from the class table instead of the supplied mask. Whenever a question gives both a first octet and an explicit prefix, the prefix always governs the network and host split.

The same caution applies to public versus private classification. Private status is a function of the documented RFC 1918 ranges, not of any class. The Class A range spans 1-126 in the first octet, but only 10.0.0.0/8 inside it is private; 11.0.0.0/8 through 126.0.0.0/8 are public. Likewise the bulk of Class B and Class C space is public, with just 172.16.0.0/12 and 192.168.0.0/16 carved out as private. Memorize the three private blocks as fixed facts and treat everything else outside the loopback, APIPA, and multicast reservations as public.

Address Recognition Drill

Drill these until classification is instant, because the exam expects sub-five-second recognition on PBQ scenarios where address type is the whole answer.

Finally, remember the practical chain of evidence APIPA gives you on a real call. A user reports "no internet," you check ipconfig, and you see a 169.254 address. That address is the system telling you it asked for a lease and got silence. The fault is never APIPA itself; it is whatever sits between the client and the DHCP server, such as a down DHCP service, an unconfigured IP helper on the router, a blocked UDP 67/68 path, a misassigned VLAN, or a dead switch port. Treat the 169.254 address as a diagnostic flag pointing upstream, not as a setting to delete.