2.5 - IP addressing: IPv4, IPv6, APIPA and DHCP
CompTIA A+ Core 1 objective 2.5 covers IP addressing. A workstation that self-assigns a 169.254.x.x (APIPA) address and cannot reach the network usually failed to get an address from DHCP. You must recognise the private IPv4 ranges - 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 - which are not routable on the internet. An IPv6 address is 128 bits long, far larger than a 32-bit IPv4 address. Devices such as servers and printers that need a fixed address should use a static IP rather than DHCP, which hands out addresses automatically. Expect scenario questions that show an address or symptom and ask what it means, whether it is private, or when static addressing is appropriate. Spotting a 169.254 address on sight instantly tells you DHCP did not respond.
169.254.x.x + no network = APIPA (DHCP failed). Private ranges = 10/8, 172.16/12, 192.168/16. IPv6 = 128 bits. Servers/printers needing a fixed IP = static (not DHCP).
Practice questions
1. How many bits long is an IPv6 address?
- 32
- 64
- 128 (correct answer)
- 256
IPv6 addresses are 128-bit (eight groups of hex), vastly expanding the address space versus 32-bit IPv4.
2. A workstation is configured with IP 169.254.18.42 / 255.255.0.0 and cannot reach the internet or other LAN hosts. The DHCP server is online. What is the MOST likely cause?
- It self-assigned an APIPA (169.254) address after DHCP failed (correct answer)
- The default gateway points to a public IP address
- DNS is misconfigured on the workstation adapter
- The subnet mask should be set to 255.255.255.0
169.254.x.x with a /16 mask is an APIPA (link-local) address, assigned automatically when a DHCP lease fails. The fix is to restore connectivity to the DHCP server (cabling, switch port, DHCP scope exhaustion).
3. Which of these is a private IPv4 address range?
- 8.0.0.0/8
- 172.32.0.0/12
- 192.168.0.0/16 (correct answer)
- 169.254.0.0/16
Private ranges: 10.0.0.0/8, 172.16–31.0.0/12, 192.168.0.0/16. 169.254 is APIPA (link-local), not private-routable.
4. When should a device use a STATIC IP rather than DHCP?
- Always, for every company laptop
- For servers/printers needing a fixed IP (correct answer)
- Never — DHCP suits every device
- Only for employee mobile phones
Servers, printers and network gear often use static IPs (or DHCP reservations) so their address never changes.
5. A /24 subnet provides how many usable host addresses?
- 256
- 254 (correct answer)
- 128
- 64
/24 = 256 addresses, minus network and broadcast = 254 usable hosts.
6. Which is true about the relationship between bandwidth and latency?
- They are exactly the same thing
- Bandwidth is capacity; latency is delay (correct answer)
- High bandwidth means zero latency
- Latency increases available bandwidth
Bandwidth = how much data per second; latency = delay. A fast link can still feel slow if latency is high (e.g., satellite).