N10-009 Question 117
Single answerPublic vs. private, Automatic Private IP Addressing (APIPA), RFC1918, Loopback/localhostA technician is troubleshooting a user's Windows laptop that cannot access the company intranet or the internet after being moved to a different office. Running ipconfig shows the laptop has an address of 169.254.44.18 with a subnet mask of 255.255.0.0 and no default gateway. The technician can successfully ping 127.0.0.1, but cannot ping the local router. Which of the following BEST explains the problem?
- A
The laptop has assigned itself an APIPA address because it could not reach a DHCP server
- B
The laptop has a valid RFC1918 private address, but the router is blocking private IP traffic
- C
The laptop has obtained a public IP address that cannot be routed on the corporate LAN
- D
The successful ping to 127.0.0.1 proves the network switch port is working correctly
Show answer and explanation
Correct answer: A
Explanation
This scenario points to a DHCP failure. In Windows and other systems using IPv4 DHCP, a host that cannot obtain an address may self-assign an APIPA address from 169.254.0.0/16, as defined for IPv4 link-local addressing. These addresses are not publicly routable and are distinct from RFC1918 private ranges, which include 10.0.0.0/8, 172.16.0.0 through 172.31.0.0, and 192.168.0.0/16 as defined in RFC 1918. The absence of a default gateway further supports APIPA rather than a valid DHCP lease. The successful ping to 127.0.0.1 confirms only that the local protocol stack is operational; loopback testing does not validate external network connectivity. Best practice would be to check physical connectivity, switch/VLAN assignment, DHCP scope availability, and whether the DHCP server or relay is reachable. Relevant references include RFC 1918 for private IPv4 addressing and RFC 3927 for IPv4 link-local addressing.
- A. Correct.
Correct. The 169.254.0.0/16 range is used for Automatic Private IP Addressing (APIPA) when a host configured for DHCP cannot obtain a lease. An APIPA address allows only limited local-link communication and does not include a default gateway, which matches the scenario. This is why the laptop cannot reach the intranet, internet, or local router.
- B. Incorrect.
Incorrect. RFC1918 private IPv4 ranges are 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. The address 169.254.44.18 is not an RFC1918 private address; it is an APIPA link-local address. The issue is failure to get DHCP information, not a router policy blocking private IP traffic.
- C. Incorrect.
Incorrect. 169.254.44.18 is not a public IP address. It is a link-local APIPA address that is not routable beyond the local segment. This option reflects a common misunderstanding between public addressing and self-assigned local addressing.
- D. Incorrect.
Incorrect. A successful ping to 127.0.0.1 only verifies that the local TCP/IP stack is functioning on the laptop. Loopback/localhost testing does not confirm switch port operation, cabling, VLAN membership, or router connectivity. Someone might choose this if they confuse host stack testing with end-to-end network testing.