Hi every body
Let say one isp has two customers, company 1 and company 2
isp assigns 10.0.0.0/8 to company 1
isp assigns 10.10.0.0/16 to company 2
isp router has the following route;
c 10.0.0.0/8 directly connected S1
c 10.10.0.0/16 directly connected S2
Let say company1 subnets it 10.0.0.0/8 as 10.10.0.0/16 and one host gets the address 10.10.10.10
let say host 10.10.10.10 sends the ping packets across the internet to some node.
Here is my understanding.
1) company 1 router connected to ISP , receives the ping packet and forwards it to isp.
2) Isp forwards the packet and eventually packet reach the destination.
3) echo reply will be sent to host 10.10.10.10 as destination address.
4) ISP receives the echo reply with destination 10.10.10.10.
5) Since the look up will be performed on the longest match, ISP router forwards the echo reply packet out of S2.
The result is host 10.10.10.10 will not receive any echo reply as it is located in company 1 , accessible by isp router's s1
is my understanding correct?
You are correct. The longest prefix is always matched first for a forwarding decision. In our scenario the 'company 1' host would never see the echo-reply.
I would hope an ISP wouldn't allocate a /8 to one company and then allocate a subordinate /16 belonging to the original /8 to another company.