Elementary Static Route Question

foreverlearning

Ping is a bidirectional packet transfer.
I issue the command ip route 0.0.0.0 0.0.0.0 <isp ip address>.

The router that I issue the command knows that he has to throw the packets to the ISP.
But how does the ISP know that it has to throw the packet back to me??

Info_Sec_Wannabe

As how I understand it, the ISP assigns a unique IP address to each client or subscriber. How they assign it is a different topic as it can be static or dynamic. 

With that said, any requests coming from you will have a source and destination IP in the request header that will be used by the ISP to determine the source of the request, being you. 

foreverlearning

If I put command ip route-static 192.168.1.0 255.255.255.0 10.0.0.1

The router knows that to reach 192.168.1.0/24, the next hop is 10.0.0.1.

But does the peer router knows how to throw the packet back?

So how can the ping be successful?

JDMurray

The IP header contains the IP address of both end-hosts in the ICMP REQUEST/RESPONSE conversation. Layer 3 preserves these source and destination IPs during each Layer 2 hop across all routers. Each router contain a table of where the source and destination IPs are located (i.e., through which router interface each IP can be found). If a router in the path is missing a route to an end-host, or an ACL is preventing ICMP from being received by an end-host, the ping response will not be received by the end-host sending the ping request.