IPv4 Classes, Private/Public, APIPA, and Loopback
Key Takeaways
- IPv4 addresses are 32-bit values written as four decimal octets, but subnet masks determine the network boundary.
- Legacy classes are useful for recognition, but modern networks use CIDR rather than classful routing.
- Private IPv4 ranges are 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16.
- APIPA addresses in 169.254.0.0/16 usually indicate a host could not obtain DHCP configuration.
- Loopback 127.0.0.0/8 tests the local TCP/IP stack and never represents a remote host.
IPv4 addressing is foundational for Network+ N10-009. The exam expects you to recognize address types, interpret masks, and troubleshoot symptoms such as APIPA addresses or incorrect default gateways.
CompTIA Network+ N10-009 uses these official domain weights: Networking Concepts 23%, Implementation 20%, Operations 19%, Security 14%, and Troubleshooting 24%. Subnetting sits mainly in Networking Concepts, but it also appears in implementation and troubleshooting scenarios.
| IPv4 concept | Meaning | Example |
|---|---|---|
| IPv4 address | 32-bit address written in dotted decimal | 192.168.10.25 |
| Octet | 8-bit decimal field from 0 to 255 | 192 is one octet |
| Subnet mask | Marks network bits and host bits | 255.255.255.0 |
| CIDR prefix | Slash notation for mask length | /24 |
| Network ID | First address in the subnet | 192.168.10.0/24 |
| Broadcast address | Last address in the subnet | 192.168.10.255/24 |
| Usable host range | Addresses between network and broadcast | 192.168.10.1-192.168.10.254 |
Legacy Classes
Classful addressing is mostly historical, but the ranges still appear in questions. CIDR is the modern method, so do not assume a mask only from the first octet unless the question explicitly asks about classes.
| Class | First octet range | Default mask | Historical purpose |
|---|---|---|---|
| A | 1-126 | /8 | Very large networks |
| B | 128-191 | /16 | Medium networks |
| C | 192-223 | /24 | Small networks |
| D | 224-239 | Not for hosts | Multicast |
| E | 240-255 | Not normal host use | Experimental or reserved |
127.x.x.x is reserved for loopback, so class A host ranges do not include it for normal addressing. 0.0.0.0 has special meanings such as unspecified address or default route, depending on context.
Private, Public, APIPA, and Loopback
| Range | Type | What it tells you |
|---|---|---|
| 10.0.0.0/8 | Private | Internal addressing, normally NATed to internet |
| 172.16.0.0/12 | Private | Includes 172.16.0.0 through 172.31.255.255 |
| 192.168.0.0/16 | Private | Common home and small office range |
| 169.254.0.0/16 | APIPA/link-local | DHCP likely failed on an IPv4 client |
| 127.0.0.0/8 | Loopback | Local TCP/IP stack testing |
| 224.0.0.0/4 | Multicast | One-to-many delivery group address |
| Public ranges | Internet-routable except reserved blocks | Assigned by ISP, cloud, or registry process |
Troubleshooting Clues
| Symptom | Likely interpretation |
|---|---|
| Host address is 169.254.23.44 | DHCP failed or unavailable |
| Host can ping 127.0.0.1 but not gateway | Local stack works; check local NIC, VLAN, cable, gateway, or address |
| Host has 192.168.1.25 but gateway is 192.168.2.1/24 | Gateway is not in the local subnet |
| Two hosts have the same IPv4 address | Duplicate IP conflict |
| Host has public IP where private was expected | Wrong DHCP scope, static misconfiguration, or exposed placement |
PBQ-Style Thinking
Scenario: A workstation shows 169.254.77.19 with mask 255.255.0.0. It cannot reach file shares or the internet. The best first checks are DHCP server availability, relay/IP helper configuration, VLAN assignment, switch port status, and local cable or wireless association. APIPA is a symptom, not the root cause.
Scenario: A printer is assigned 192.168.10.50/24 with default gateway 192.168.11.1. It can talk to some local hosts but cannot leave the subnet. The gateway is outside the printer subnet. Fix the gateway or address plan so the default gateway is reachable on the local network.
Address Recognition Drill
| Address | Classification |
|---|---|
| 10.44.3.2 | Private IPv4 |
| 172.20.5.6 | Private IPv4 |
| 172.40.5.6 | Public IPv4 unless otherwise reserved by context |
| 192.168.100.10 | Private IPv4 |
| 169.254.8.9 | APIPA/link-local |
| 127.0.0.1 | Loopback |
| 224.0.0.5 | Multicast |
A workstation has IPv4 address 169.254.18.9 and cannot reach network resources. What is the most likely issue?
Which IPv4 range is private?
Which addresses should not be assigned as normal remote host addresses? Choose two.
Select all that apply