Elementary Static Route Question

foreverlearningforeverlearning Member Posts: 42 ■■■□□□□□□□
Ping is a bidirectional packet transfer.
I issue the command ip route <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_WannabeInfo_Sec_Wannabe Member Posts: 428 ■■■■□□□□□□
    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. 
    X year plan: (20XX) OSCP [ ], CCSP [ ]
  • foreverlearningforeverlearning Member Posts: 42 ■■■□□□□□□□

    If I put command ip route-static

    The router knows that to reach, the next hop is

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

    So how can the ping be successful?

  • JDMurrayJDMurray Admin Posts: 13,025 Admin
    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.
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