ICMP Error Messages

Below, I've captured some anecdotal reasons you will see some common ICMP errors. They are NOT absolutes, but just things to think about should you encounter them.

Network unavailable typically means there is no route to the network.

Host unreachable is usually one of two reasons:

• Layer 1 is broken, host physically offline.
• A security feature is preventing access to the host (firewall, proxy, etc).

Port Unreachable is usually one of two reasons:

• A security feature is preventing access to that port (firewall, proxy, etc).
• A service is likely not running on that port*

TTL Exceeded

• Something is unusual about the route to the host that makes the path quite long.

Reassembly Time Exceeded

• I have never encountered this, however a severely degraded connection with multiple varying MTUs could theoretically cause this.