Networking, not just firewalls, is about looking at a packet, opening it up, and either passing it along to the next link in the chain, or making a change to (t

Networking, not just firewalls, is about looking at a packet, opening it up, and either passing it along to the next link in the chain, or making a change to (typically) the header and sending it along. There's a TTL or Time To Live value to make packets eventually die, otherwise, packets could potentially move from router to router forever. So at a minimum, your firewall, or router, rewrites the header to Decrement the TTL by one with every hop. Which spawns a really interesting thought experiment that results in the creation of the traceroute utility.