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Why does my ping fail even though IP addresses are correct?

22maya2n2 · posted · 92 views

I set up two PCs with IPs on the same subnet and connected them through a router. The IPs are definitely correct but ping times out. What should I check? I feel like I'm missing something obvious. Any troubleshooting checklist would be really helpful.

troubleshootingicmpconnectivity
  • 9 times out of 10 it's either a missing default gateway on the PC or the interface is admin-down. Always check show ip interface brief first — look for "administratively down" in the Status column. Also make sure both PCs have gateways pointing to the router. — NivSa,
  • Also check if there's an ACL blocking ICMP. I spent an hour once troubleshooting a failed ping before realizing I had an extended ACL on the outbound interface that was denying all traffic. The implicit deny at the end catches everything you didn't explicitly permit. — itayitayy,
  • It was the default gateway! Both PCs had IPs but I forgot to set the gateway pointing to the router interface. show ip interface brief showed everything as up/up so I assumed it was fine. Lesson learned — always check the gateway on endpoints. — 22maya2n2,

1 Answer

Correct IPs are only half the story — ping needs Layers 1-3 all working plus ARP. Walk it top-down:

  1. Subnet mask match — both hosts must agree they're on the same subnet. 10.0.0.1/24 and 10.0.0.2/25 look 'correct' but disagree about who's local, so one side tries to route.
  2. Default gateway — pinging a different subnet with no gateway (or the wrong one) fails silently. Ping your own gateway first to isolate.
  3. ARP — if ARP can't resolve the next hop's MAC, the ICMP never leaves. Run show arp / check the ARP table; an incomplete entry points to L1/L2.
  4. Layer 1/2 — link up? Right VLAN? If the two PCs are on different VLANs, or the trunk isn't carrying that VLAN, they'll never reach each other.
  5. ACLs / port-security — an ACL denying ICMP, or a shut/err-disabled port, drops the ping even with perfect addressing.

Start with show ip interface brief (is the interface up/up?), then ping the gateway, then check the ARP table. In the lab, run the ping and watch the cable — if the ARP request goes out but no reply comes back, your problem is L2/VLAN, not IP.

Shira Sivroni · answered
  • Great checklist. One more classic: same subnet, correct masks, but the two hosts landed in different VLANs on the switch — L2 broadcast domains don't connect, so ARP never completes. `show mac address-table` and `show vlan brief` catch it fast. — ben31833,

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