N10-009 Question 116
Single answerPublic vs. private, Automatic Private IP Addressing (APIPA), RFC1918, Loopback/localhostA technician is troubleshooting a user workstation that cannot access internal file shares or the internet. Running ipconfig shows the IPv4 address 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 at 192.168.10.1. Which of the following is the MOST likely cause of the problem?
- A
The workstation failed to obtain an address from DHCP and assigned itself an APIPA address
- B
The workstation received a valid public IP address that is blocked by RFC1918 filtering
- C
The loopback test shows the NIC is disconnected from the local subnet
- D
The router is misconfigured because 192.168.10.1 is not a valid private IPv4 address
Show answer and explanation
Correct answer: A
Explanation
This scenario combines several core Network+ objectives: APIPA, RFC1918 private addressing, public versus private addressing, and loopback testing. An address of 169.254.44.18 with a 255.255.0.0 mask and no default gateway strongly indicates APIPA self-assignment after DHCP failure. In Microsoft environments, this commonly occurs when the DHCP server is unavailable, the client cannot reach it, or DHCP traffic is being blocked. A successful ping to 127.0.0.1 confirms the local TCP/IP stack is loaded and functioning, but it does not confirm valid network-layer connectivity beyond the host. The router address 192.168.10.1 is part of the RFC1918 private 192.168.0.0/16 range and is valid for internal use. Relevant references include RFC 1918 for private IPv4 address space and RFC 3927 for IPv4 link-local addressing behavior.
- A. Correct.
Correct. The 169.254.0.0/16 range is Automatic Private IP Addressing (APIPA), also called link-local addressing in IPv4. Windows clients commonly self-assign an address in this range when they cannot contact a DHCP server. Because APIPA does not provide a default gateway, the host typically cannot reach other subnets or the internet and often cannot communicate properly with devices on a differently addressed LAN such as 192.168.10.0/24.
- B. Incorrect.
Incorrect. 169.254.44.18 is not a public IPv4 address. It is an APIPA/link-local address used for local communication when DHCP fails. RFC1918 defines private ranges such as 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16, but 169.254.0.0/16 is not one of them. The issue is not public addressing or RFC1918 filtering.
- C. Incorrect.
Incorrect. Successfully pinging 127.0.0.1 verifies the local TCP/IP stack is functioning on the host. It does not prove end-to-end LAN connectivity, and it does not by itself indicate the NIC is disconnected. A failed ping to the router in this case is more consistent with incorrect local addressing due to DHCP failure than with the meaning of the loopback test.
- D. Incorrect.
Incorrect. 192.168.10.1 is a valid private IPv4 address within the RFC1918 range 192.168.0.0/16. Using that address for a local router interface is common in small and medium networks. The problem is not that the router's address is invalid.