Computer's IP address goes "Bad". No internet access. Local network still works fine. Can ping any other computer on the network. I can ping the DNS server. Pinging external domains resolve but time out. Nslookup responds correctly. Other computers on the network have internet access. If I change a computer's MAC address to the same MAC address as the computer with the "bad" ip address (and turn that computer off), it pulls the same IP address and experiences the same problem. We are using DeepNines web filter. What could be possibly causing this intermittent problem?
Sounds like two pieces of equipment on the same switch with the same MAC address. This happened to me several years ago.
It sounds like somewhere along the line something firewall-like is blocking the IP.
One thing you can do to determine where it breaks down is to run a traceroute (on windows,
tracert), and then ping each server in the path; this may get you closer to where the problem lies.
It's also possible that a misconfigured NAT device is doing something odd to the traffic, or that you have a subnetting issue (this will happen if the subnet mask on the gateway is incorrect and it thinks the host isn't in its subnet, sometimes).