Newbie TTL Expired in Transit question

I know this has to be something simple but I'm not sure what. We have a simple config on our Internet router supplied by our ISP. We had 2 sets of IP's assigned to us...for example, (router serial interface IP) and for hosts. We have static nats for the 75.75.75.x ip's and a single IP Route command pointing everything out the Serial interface. When we ping the serial interface, it responds fine. When we ping any of the 75 addresses, we get the "TTL Expired in Transit" reply...tracert shows a continuous loop between our serial interface and the next hop to our ISP. But...we can still access whatever is running on the hosts (webserver, VPN, email...etc). What am I missing here? Shouldn't the pings be either timing out or replying depending on our firewall config? And why does the ping expire but port 80, 25..etc. are fine? It's not hurting anything...more of a curiosity question.

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The Other Mike
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what device is doing NAT - your ISP router or your firewall behind the router? take a look at

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will see how it jumps from outside to you

the answer to your question is another question "how it's configured to travel from the INSIDE?"

Roman Nakhmanson

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