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Subnetting ? I can ping different net !

Posted on 2001-08-23
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Last Modified: 2008-03-04
It strange huh..
I have a PC with IP : 10.1.0.19 mask 255.255.255.128 and other PCs with IP : 10.1.0.X mask 255.255.255.0

The computers can ping each other. How it would be ? There is subnetting I put there to difference between this computers. But they can ping each other. Any explaination ?
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Question by:Imelda
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by:rid
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I think that is quite all right. I mean, I think ping works that way. Is this a problem? I think you'll need a firewall or something to hide the computers from each other.
Regards
/RID
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by:vsamtani
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Do you have gateways defined for the two machines?

If not, then although they are on different subnets, they will have no option but to send the ping packet out on the local segment. The other pc does happen to be on the local segment, so it will reply.

If you want to isolate the two subnets from each other, you really should put them on different cables. And then you would put a router in between in order to connect them up again.

Vijay
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jwalsh88 earned 50 total points
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Here,s how this works lmelda.  

The first subnet you have is 10.0.1.0 255.255.255.128
That means that you are using the network range of
10.0.1.0-10.0.1.127.  Now, 10.0.1.0 is the network and
10.0.1.127 is the broadcast address.  your machine is
using IP address 10.0.1.19 which means he will send out an
arp request for the mac address of any machine on his
subnet.  Which is 10.0.1.1-10.0.1.126.  If the machine
doesn't fall in that range he will look in his routing
table to see where he is supposed to send these packets
(usually a default route/gateway).  He only uses his
subnet mask to decide which PCs are on his local subnet
and which ones he needs to send to a router.  Your other
subnet is 10.0.1.0 255.255.255.0.  That means that you are
using the network range of 10.0.1.0-10.0.1.254.  If you
notice you have overlapping subnets here.  So, PCs on your
first subnet will be able to send to any machine on your
second subnet with and IP address of 10.0.1.1-10.0.1.126
as long as they are on the same physical LAN segment.  The
reason this happened was that you subnetted on one side
but not the other side.  This is what you should have done.

Make one network 10.0.1.0 255.255.255.128 and you can use
IP addresses 10.0.1.1-10.0.1.126 on your machines.  Then,
you make the other network 10.0.1.128 255.255.255.128 and
you can use IP addresses 10.0.1.129-10.0.1.254.  That will
give you 126 addresses for each subnet and they can't talk to each other.  If that's not enough address then subnet differently like this:
10.0.1.0 255.255.255.0 and 10.0.2.0 255.255.255.0.
or even like this:
10.0.0.0 255.255.0.0 and 10.1.0.0 255.255.0.0
that will give you these ranges respectively:
10.0.0.1-10.0.255.254
10.1.0.1-10.1.255.254
**Thats alot of IP addresses**

Anyways if you have your example on the same network (Physically)
Then you are seeing normal activity, you just have your subnets wrong.
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Author Comment

by:Imelda
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jwalsh88,You said there is address overlapping subnet. Do you mean IP address 10.1.0.1 with mask 255.255.0.0 will detected as a conflicted IP address with 10.1.0.1 with mask 255.255.255.0 ?
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Expert Comment

by:rid
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I don't think you can have two machines with the *same* address on the physical network - irrespective of netmask. It would seem you need to give the machines that should NOT be able to communicate addresses in different ranges, as outlined by jwalsh88. The netmask sets the "borderline" and the addresses must be in the ranges on either side of this line for communication not to work.
Regards
/RID
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Expert Comment

by:jwalsh88
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The only purpose for the subnet mask is so that the machine knows when to look into it's routing table for a route to a machine outside his network range.  And to set the broadcast address for his network in his routing table.
When I said overlapping I meant that there were two networks defined and one of the ranges of one network overlaps the range of another.  rid is correct, you can never have two devices with the same IP address, regardless of subnet mask, on the same physical wire.  The will here each other and conflict.
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Expert Comment

by:deschneider
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Jwalsh88 and rid sum it up pretty well. Your network should under normal circumstances use only one subnet mask. THat way you are able to segment traffic.
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by:jwalsh88
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You can use multiple subnet mask lengths but that takes alot of planning and you really need to know what you are doing.  In most cases always use the same length of masks.
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Author Comment

by:Imelda
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Now I have known the problem that I faced. I decide to accept jwalsh88 as answer. Thank you for all you guys. :)
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