Solved

class c subnet on class b subnet

Posted on 2010-11-16
9
598 Views
Last Modified: 2012-05-10
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.
0
Comment
Question by:tad91030
9 Comments
 
LVL 11

Assisted Solution

by:MajorBigDeal
MajorBigDeal earned 166 total points
ID: 34151780
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.
0
 
LVL 14

Expert Comment

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.
0
 
LVL 14

Accepted Solution

by:
Otto_N earned 168 total points
ID: 34154284
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.
0
Industry Leaders: We Want Your Opinion!

We value your feedback.

Take our survey and automatically be enter to win anyone of the following:
Yeti Cooler, Amazon eGift Card, and Movie eGift Card!

 
LVL 2

Assisted Solution

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

0
 

Author Comment

by:tad91030
ID: 34158843
any suggestions for routers. we have an rv082 but it only allows class c mask.
0
 
LVL 11

Expert Comment

by:MajorBigDeal
ID: 34167809
That is a problem.  You'll either have to use a second router or else rework your network. Sorry!
0
 
LVL 11

Expert Comment

by:MajorBigDeal
ID: 34769641
Any Luck?
0
 
LVL 69

Expert Comment

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.
0

Featured Post

Free Tool: Path Explorer

An intuitive utility to help find the CSS path to UI elements on a webpage. These paths are used frequently in a variety of front-end development and QA automation tasks.

One of a set of tools we're offering as a way of saying thank you for being a part of the community.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Join Greg Farro and Ethan Banks from Packet Pushers (http://packetpushers.net/podcast/podcasts/pq-show-93-smart-network-monitoring-paessler-sponsored/) and Greg Ross from Paessler (https://www.paessler.com/prtg) for a discussion about smart network …
For many of us, the  holiday season kindles the natural urge to give back to our friends, family members and communities. While it's easy for friends to notice the impact of such deeds, understanding the contributions of businesses and enterprises i…
After creating this article (http://www.experts-exchange.com/articles/23699/Setup-Mikrotik-routers-with-OSPF.html), I decided to make a video (no audio) to show you how to configure the routers and run some trace routes and pings between the 7 sites…
This video gives you a great overview about bandwidth monitoring with SNMP and WMI with our network monitoring solution PRTG Network Monitor (https://www.paessler.com/prtg). If you're looking for how to monitor bandwidth using netflow or packet s…

713 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question