what is the implication of the setup below in real life ? (CIDR)

[SJ K]

Hi all,

With CIDR/VLSM in place, networks can now be are actually subsets of a larger network originally ->

e.g. 192.168.0.0/16 network contain 192.168.0.1 to 192.168.255.254 hosts

and 192.168.1.0/24 can be originally part of the network above.
 

I have done up a simple lab below and it seems even though the interface on individual router belong to different "network". (as long as they thought the destination is in the same network, communication can still be established)

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R1#show ip int
FastEthernet0/0 is up, line protocol is up
  Internet address is 192.168.3.1/24
  Broadcast address is 255.255.255.255


R2>show ip int
FastEthernet0/0 is up, line protocol is up
  Internet address is 192.168.3.2/25
  Broadcast address is 255.255.255.255


R1#ping 192.168.3.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.3.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 16/35/68 ms


R2>ping 192.168.3.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.3.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 16/33/48 ms
R2>

R1 and R2 are connected via Fe0/0 interface on each router. There are no gateway of last resort set.

As  you can see, if R1 and R2 belong to 2 different administrators and accidentally 1 of them have set the network wrongly to overlap with another.

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q1) With this subsetting in place, how does ISP or company with medium network size prevent overlapping of assigned networks ? I believe there are bound to be.

q2) What will be the implication in this in real life ? Right now, both side are still pingable though. How about if there is a 3rd router connected to both of them. On which router will R3 route to for 192.168.3.3  ?

Regards,
Noob

[devils_advocate]

An ISP will use Public IP addresses which they control and assign so they do not overlap.

A medium/large company will plan their IP address allocations so they do not overlap, you will find a lot of companies use 'easily management' subnet masks like /24 and /22. We use /24 for all subnets, sure it wastes some address but we are nowhere near large enough for it to become an issue. 

Are your routers connected back to back via the Fa0/0 link?
If so, all you have done wrong is mismatch the subnet masks but it will still work because the /24 subnet mask also includes the hosts from the smaller /25 subnet.

/24 incorporates 192.168.3.1 through 192.168.3.254 as assignable hosts.
/25 incorporates 192.168.3.1 through 192.168.3.127 as assignable hosts.

As you can see, 192.168.3.1 and 192.168.3.2 are in both so it will work fine.
Its not technically correct and its definitely not good practice but it will work. 

[SJ K]

Hi Devil,

Thanks for reverting but human errors do occur even regardless of how big the corporate is isn't it ?  (Just guessing)

q1) as mentioned, such configuration above are technically wrong, but workable, how do we detect such issues then in real life ? Are there any implications with such setup such as route advertisement etc ?

q2) In the event of human error (taking the example above), what is there is another router R3 connected to both of these 2 routers. Which path will R3 select if there's a need to reach 192.168.3.3 ?

Regards,

Noob