Two Ethernet LANs are connected by a router (TCP/IP). A newly added client can only access hosts on its own side of the router, while all other hosts intercommunicate normally. What is the most likely cause?

Introduction / Context:
When a single new client can reach only same-subnet hosts but not remote subnets across a router, a layer-3 addressing or masking problem on that client is the prime suspect. Routers forward between different IP networks; incorrect masks cause wrong on-link/off-link decisions and failed default-gateway use.

Given Data / Assumptions:

• Two Ethernet networks connected by a router.
• All other hosts communicate properly across the router.
• The new client can reach only local hosts.
• Physical links and router are functioning for everyone else.

Concept / Approach:

The subnet mask determines which IPs are considered local. If the mask is too wide or mismatched, the client may believe remote hosts are on its local subnet, sending ARP requests instead of packets to the default gateway. Those ARPs fail across the router boundary, breaking remote connectivity, while local connectivity remains fine.

Step-by-Step Solution:

Verification / Alternative check:

Packet capture on the client shows ARPs for remote IPs when the mask is wrong; after fixing the mask, traffic goes to the gateway MAC and routing succeeds.

Why Other Options Are Wrong:

Termination/repeater faults: Layer-1/2 issues would affect multiple hosts, not only the new client.

Defective router NIC: Others would fail too; symptoms are isolated to one client.

None of the above: The wrong mask neatly explains the behavior.

Common Pitfalls:

Blaming DNS; DNS may resolve, but reachability fails due to incorrect on-link decision from the bad mask.

Final Answer:

Incorrect subnet mask on your client computer