Re: Possible IP conflict, bad math, or over thinking. Need clarification.
- From: Trendkill <jpmason@xxxxxxxxx>
- Date: Fri, 30 Nov 2007 14:00:09 -0800 (PST)
On Nov 30, 4:33 pm, Chris <chris.bl...@xxxxxxxxx> wrote:
Hello all,
I'm looking over some subnet math and my brain is telling me there is
a problem, but the force is telling me otherwise.
My main question is: Can you have these two networks living together
and not overlap each other? Part of me says yes because the masks are
different, but the other part says no because the IP's would be
identical and create a conflict on the network. Am I over thinking
this?
non-dhcp range: 10.30.23.128/25
dhcp range: 10.30.23.128/26
Why someone is doing it this way is beyond me; I'm just trying to
figure out the logic. Here is the math that got me to the point I'm
at. Am I on track with the thinking and the math?
range non-dhcp: 10.30.23.128/25
23.128 in binary
128 64 32 16 8 4 2 1 || 128 64 32 16 8 4 2 1
0 0 0 1 0 1 1 1 1 0 0 0 0 0 0 0
Subnet mask /25 in binary.
128 64 32 16 8 4 2 1 || 128 64 32 16 8 4 2 1 || 128 64 32 16 8 4 2 1
|| 128 64 32 16 8 4 2 1
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 0 0 0 0 0 0 0
255 255 255 128
Multiply last two octets of the ip address and mask together.
128 64 32 16 8 4 2 1 || 128 64 32 16 8 4 2 1
0 0 0 1 0 1 1 1 1 0 0 0 0 0 0 0
1 1 1 1 1 1 1 1 1 || 0 0 0 0 0 0 0 || is visual guide
for the network separation
0 0 0 1 0 1 1 1 1 || 0 0 0 0 0 0 0 = Network ID .
23.128
0 0 0 1 0 1 1 1 1 || 0 0 0 0 0 0 1 = First Usable IP .
22.129
0 0 0 1 0 1 1 1 1 || 1 1 1 1 1 1 0 = Last usable IP .
23.254
0 0 0 1 0 1 1 1 1 || 1 1 1 1 1 1 1 = Broadcast .
23.255
Valid range is: 10.30.23.129 - 10.30.23.254
--------------------------------------------------------------------------------
DHCP range: 10.30.23.128/26
23.128 in binary
128 64 32 16 8 4 2 1 || 128 64 32 16 8 4 2 1
0 0 0 1 0 1 1 1 1 0 0 0 0 0 0 0
Subnet mask /26 in binary.
128 64 32 16 8 4 2 1 || 128 64 32 16 8 4 2 1 || 128 64 32 16 8 4 2 1
|| 128 64 32 16 8 4 2 1
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 0 0 0 0 0 0 0
255 255 255 192
Multiply last two octets of the ip address and mask together.
128 64 32 16 8 4 2 1 || 128 64 32 16 8 4 2 1
0 0 0 1 0 1 1 1 1 0 0 0 0 0 0 0
1 1 1 1 1 1 1 1 1 1 || 0 0 0 0 0 0 || is visual
guide for the network separation
0 0 0 1 0 1 1 1 1 0 || 0 0 0 0 0 0 = Network ID .
23.128
0 0 0 1 0 1 1 1 1 0 || 0 0 0 0 0 1 = First Usable
IP .22.129
0 0 0 1 0 1 1 1 1 0 || 1 1 1 1 1 0 = Last usable IP .
23.191
0 0 0 1 0 1 1 1 1 0 || 1 1 1 1 1 1 = Broadcast .
23.192
Valid range is: 10.30.23.129 - 10.30.23.191
The short answer is that this is not a good idea whatsoever. Since
your router IP is the same for both ranges, and presuming you were
using the larger mask to ensure you have coverage for both networks,
this could work if the mask was only different on the nodes
themselves, although broadcast would be broken for your smaller
network nodes. That being said, you definitely cannot have the same
network configured on two different routers (you would have a routing
problem, as the router IPs would be the same but would need to be
split which is impossible), and most routers will not allow you to
have the same IP configured on two different physical or logical
interfaces.
In short, your first instinct is right, you CANNOT have overlap
whatsoever with the same IPs being used by two different networks,
regardless of mask. You could probably have some really odd
configurations with masks being different on boxes themselves, and
things looking like they are working ok, although certain services
like broadcast, etc, may not be working properly and you just don't
know it because the boxes don't use those services.
You could use a secondary interface on the router and split the
networks up that way. But in short answer to your question, no you
cannot. You technically would have 128 - 192 assigned to both
networks, and only the bigger network will have 192-255. Either way,
the router can't have both without a secondary interface, and without
that, broadcast would be broken for your smaller network nodes. Your
math is fine, the overlap is what will confuse things.....
.
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