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class c subnet on class b subnet

Posted on 2010-11-16
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Can a class c subnet be on the same network as a class b subnet.
class b address 10.10.1.1 255.255.0.0 ( not using the class c addresses )
class c address 10.10.10.1 255.255.255.0
i think the answer is no but wanted to see if anyone had used this configuration.
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Question by:tad91030
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by:MajorBigDeal
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Yes, you can do this because the most specific route will win.  In other words, routes do not have to be defined to be completely mutually exclusive (although I think it is better if they are).  In those cases where more than one subnet matches, the router will attempt to figure out which route is the most specific and use that one.
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by:Otto_N
ID: 34153956
To add to MajorBigDeal's comment:  Yes, you can, but it just open up opportunities for confusion.  Also, if you want devices in the separate subnets to talk to each other, it will not work.
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Otto_N earned 168 total points
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Just to explain, if you have overlapping subnets, why communication between the subnets will fail. Let's use the subnets in the example:
Subnet1 - 10.10.0.0/16
Subnet2 - 10.10.10.0/24

If you have a host in each subnet (Host A with IP 10.10.1.5 in Subnet1, and Host B with IP 10.10.10.15 in Subnet2), and A want to send an IP packet to B, the following will happen:
i) A will query its routing table and see that, to reach 10.10.10.15, it needs to use a specific interface, let's say eth0.
ii) The IP stack in host A will next check if the destination address is in the same subnet than eth0.  To do this, it compares the result of the bit-wise AND of the IP address and subnet mask of eth0 (10.10.1.5 AND 255.255.0.0 = 10.10.0.0 for this example) with the bit-wise AND of the destination IP address and the subnet mask of eth0 (10.10.10.15 AND 255.255.0.0 = 10.10.0.0). If it is the same, it send it to Layer2 for direct delivery; if not, it send it to Layer2 for delivery to the default gateway.

Since, from host A's point of view, eth0 is in the same subnet as host B, A will issue an ARP request for B's MAC address (assuming we're using Ethernet).  But B will not receive this broadcast, since it is in a different segment, and A will not receive a response.  Even if you configure proxy-ARP on your router, it will not help, since the router is also configured with the 255.255.0.0 mask for the interface in segment1.

This can be fixed with a static route on host A, directing traffic destined for Subnet2 to the default gateway, but this is a manual process that must be implemented on every host that need to communicate with Subnet2, and would not be recommended.
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by:pmanno
pmanno earned 166 total points
ID: 34157199
Just as a side note, classes pertain to IP addresses, not subnets.  What you have is a class A IP address that you are subnetting.  By subnetting you are not changing the class.  the class is determined by the location of highest order 1s in the first octet.  For example:

Class A First Octet = 0XXX XXXX
Class B First Octet = 10XX XXXX
Class C First Octet = 11XX XXXX

Ref: http://en.wikipedia.org/wiki/Classful_network

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by:tad91030
ID: 34158843
any suggestions for routers. we have an rv082 but it only allows class c mask.
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by:MajorBigDeal
ID: 34167809
That is a problem.  You'll either have to use a second router or else rework your network. Sorry!
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by:MajorBigDeal
ID: 34769641
Any Luck?
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by:Qlemo
ID: 35015601
This question has been classified as abandoned and is being closed as part of the Cleanup Program. See my comment at the end of the question for more details.
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