If the business shares or duplicates the ISP-assigned public IP address, routing instability and conflicts will occur. Pinging a public IP like 8.8.8.8 may work (since ICMP can bypass certain conflicts), but browsing websites (which requires stable sessions and return traffic) will fail intermittently.
A. If the default gateway were incorrect, no external connectivity would work at all.
B. VLAN mismatch is an internal issue, not affecting ISP routing.
C. Subnet mask misconfiguration would prevent consistent routing but usually blocks ping too.
References (CompTIA Network+ N10-009):
Domain: Network Troubleshooting — Internet connectivity issues, ISP IP conflicts.
