CompTIA Network+ (N10-009)Network TroubleshootingMedium
A user in a branch office can reach servers on the local subnet but cannot access any resources outside the subnet, including the internet. The user's IP configuration shows an address of 192.168.10.55/24 and a default gateway of 192.168.11.1. What is the MOST likely cause of this issue?
- AThe DNS server address is incorrect
- BThe DHCP lease has expired
- CThe subnet mask is too long
- DThe default gateway is on a different subnet than the host
Show answer & explanationAnswer & explanation
Correct answer: D. The default gateway is on a different subnet than the host
With a /24 mask, 192.168.10.55 belongs to the 192.168.10.0 network, but the gateway 192.168.11.1 is on the 192.168.11.0 network. Since the gateway is unreachable on a different subnet, the host cannot route traffic beyond its local subnet, though local subnet communication still works.
Why the other options are wrong
- A. DNS issues would prevent name resolution but not affect all outside traffic including IP-based access.
- B. An expired lease would typically result in an APIPA address, not a static-looking mismatched configuration.
- C. The /24 mask is standard and not too long; the problem is the mismatched gateway subnet.
Gateway Subnet Mismatch
A misconfiguration where the default gateway address falls outside the host's own subnet, preventing the host from routing traffic to other networks.
- Gateway must be in the same subnet as the host to be reachable
- Symptom: local traffic works, but no external/internet access
- Fix by correcting gateway IP or host's subnet mask
Memory trick: The gateway must live on your street (subnet), or you can't leave the neighborhood.