is my understanding correct? ipv4 assignment

Answered Question
Aug 29th, 2010

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?

I have this problem too.
0 votes
Correct Answer by gatlin007 about 6 years 4 months ago

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.



Chris

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Correct Answer
gatlin007 Sun, 08/29/2010 - 16:26

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.



Chris

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