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Difficulty: Very hardTroubleshooting Routing, Default Gateways, and ACLs

A systems administrator is configuring a newly deployed Linux server assigned the IPv4 address 172.31.88.45/22172.31.88.45/22 (subnet mask 255.255.252.0255.255.252.0). The server can successfully ping other local hosts within its local network segment, but fails to reach any external subnets or remote application servers. The administrator inspects the host network interface and routing table using standard command-line tools:

eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 172.31.88.45 netmask 255.255.252.0 broadcast 172.31.91.255

Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
0.0.0.0 172.31.92.1 0.0.0.0 UG 100 0 0 eth0
172.31.88.0 0.0.0.0 255.255.252.0 U 0 0 0 eth0

Which of the following identifies the root cause of the remote connectivity failure?

  1. The configured default gateway IP address (172.31.92.1) resides outside the server's local IPv4 subnet range.Answer
  2. B
    The broadcast address 172.31.91.255 overlaps with usable host IPs, preventing default route kernel activation.
  3. C
    An inbound access control list rule on eth0 is dropping return traffic due to an implicit deny all statement.
  4. D
    The host interface eth0 requires an explicit 802.1Q VLAN tagging ID configuration to reach off-subnet gateways.

Answer

The configured default gateway IP address (172.31.92.1) resides outside the server's local IPv4 subnet range.
For a host to communicate with remote networks, its default gateway must be on the same local IP subnet. The host is configured with 172.31.88.45/22172.31.88.45/22 (255.255.252.0255.255.252.0), which defines the local subnet range as 172.31.88.0172.31.88.0 to 172.31.91.255172.31.91.255. The configured default gateway of 172.31.92.1172.31.92.1 is part of the 172.31.92.0/22172.31.92.0/22 network. Because it resides outside the local subnet boundary, the server cannot send Layer 2 ARP requests directly to reach the gateway's MAC address, preventing all traffic destination routes outside the local subnet from establishing.

Step-by-Step Solution

1
Calculate the subnet boundary for the host IP address 172.31.88.45 with a /22 prefix (netmask 255.255.252.0).
The block size is 256 - 252 = 4 in the 3rd octet. Subnet base is 172.31.88.0/22.
Determining the subnet boundary reveals the range of valid IP addresses on the local broadcast domain.
2
Identify the usable IP address range and broadcast address for the 172.31.88.0/22 network.
Network ID: 172.31.88.0, Usable Hosts: 172.31.88.1 through 172.31.91.254, Broadcast ID: 172.31.91.255.
A default gateway must reside within the host's local usable IP range to answer Address Resolution Protocol (ARP) requests.
3
Compare the configured default gateway IP (172.31.92.1) against the local subnet boundaries.
172.31.92.1 is in the next subnet (172.31.92.0/22).
Because 172.31.92.1 is on a different IP subnet, the host cannot perform ARP resolution for its default gateway, failing all off-subnet traffic forwarding.

Key Concept

Default Gateway Subnet Mismatch
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Topics

Troubleshooting Routing, Default Gateways, and ACLs

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