Usage of CIDR 30

I have noticed when there is Point to point link between 2 Devices, they use Ip address with /30
example :
R1 interface is directly  connected to R2 interface
R1 interface =10.10.10.1/30 and R2 interface= 10.10.10.2/30
R1 and R2 are in the Network 10.10.10.0/30


Per some articles , they say it saves IP addresses when you use /30.
What I need to understand Is , can we still  use the Network 10.10.10.0 in other Networks behind R1 and R2 but  with different Mask.  if we use any Mask smaller than /30, I believe  10.10.10.1 and 10.10.10.2 will still be included.

So , if I am not wrong the 10.10.10.0  will not be used in any other Network ...

Please clarify this confusion for me.

Thank you
jskfanAsked:
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Tim PhillipsWindows Systems AdministratorCommented:
I would really discourage using overlapping IP schemes.  It would cause trouble because the router wouldn't know which interface to use for which addresses when resolving.  Like, if you ping 10.10.10.3 it would either broadcast on the P2P network or ping a host on the LAN side, but probably not both.  Really not a good idea.  Further, if you have a gateway on the router for 10.10.10.2 then your computers would possibly never make it to the internet because the router would think that is a local address.
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jskfanAuthor Commented:
So,  it would not matter if you use 10.10.10.0/30 or 10.10.10.0/24 on the point to point ?
Since you are going to loose that Network 10.10.10.0

I would say in this case if I use 10.10.10.0/20 in point to point, then in other Networks I will use  for instance10.10.10.0/22, that might not be an overlapping ?? I could be wrong ...
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Tim PhillipsWindows Systems AdministratorCommented:
Um, I think there is some confusion here.  Let's try some examples:

Here is one network: 10.10.10.0/30
  Network: 10.10.10.0
  First usable address: 10.10.10.1
  Last usable address: 10.10.10.2
  Broadcast address: 10.10.10.3

Here is another network that DOES NOT overlap the other: 10.10.10.4/30
  Network: 10.10.10.4
  First usable address: 10.10.10.5
  Last usable address: 10.10.10.6
  Broadcast address: 10.10.10.7

Why don't you just use a private IP address network for the P2P and another for the LAN?  Routing should take care of connectivity if it is setup right.
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Don JohnstonInstructorCommented:
R1 interface is directly  connected to R2 interface
R1 interface =10.10.10.1/30 and R2 interface= 10.10.10.2/30
R1 and R2 are in the Network 10.10.10.0/30
This is correct.
What I need to understand Is , can we still  use the Network 10.10.10.0 in other Networks behind R1 and R2 but  with different Mask.
No.
So , if I am not wrong the 10.10.10.0  will not be used in any other Network ...
That is correct.
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Fred MarshallPrincipalCommented:
Consider this:
Conceptually divide the address space into smaller subnets.
/30 subnets have 2 IP addresses each.
/24 have 254 each.

The /24 subnet can be split up into 256/4 = 64 /30 subnets or some smaller number of a variety of subnet of various sizes.

It sounds like you want one bigger subnet in what remains after using up the 4 /30 addresses.
(It's 4 because you're using up 10.10.10.0 to 10.10.10.3 with .1 and .2 the usable addresses and .0 and .3 the network and broadcast addresses).

You didn't really say that you wanted the others to be in a separate network.
/30 won't "save" IP addresses in any substantial way.
Here's why.  And, I hope this helps answer your question:

Since you have /30 and I'm assuming that you might otherwise want /24 for everything else, the particular addresses you've used up take up the bottom half of that /24 space in the following context:

10.10.10.0 /30 >>10.10.10.0 - 10.10.10.3
10.10.10.0 /29 >> 10.10.10.0 - 10.10.10.7
10.10.10.0 /28 >> 10.10.10.0 - 10.10.10.15
10.10.10.0 /27>> 10.10.10.0 - 10.10.10.31
10.10.10.0 /26 >> 10.10.10.0 - 10.10.10.63
10.10.10.0 /25 >> 10.10.10.0 - 10.10.10.127
10.10.10.0 /24 >> 10.10.10.0 - 10.10.10.255

Note that these ALL start at 10.10.10.0

We cannot carve out a subnet of size 256 and still have a separate subnet of size 4
The best we can do is carve out a subnet of size 128 if it's to be a separate subnet in the same range.
10.10.10.128 /25 >>10.10.10.128 - 10.10.10.255

So, you can have a size 4 subnet in the bottom half and a size 128 subnet that is the top half and not have them overlap.

Or, you can just have two separate subnets of any size you like because with private IP addresses there are plenty to "waste" and saving them does nothing of value.  If they are too big AND if you add many, many hosts then you could have traffic problems but that's not really an addressing / subnetting question.

There are some "tricks" you can play if you are bold enough:

Say you set up a network 10.10.10.0 /24
You set up all the hosts (except 2 of them) to have addresses in the range 10.10.10.8 - 10.10.10.254 (reserving 10.10.10.1 - 10.10.10.7)
You set up the gateway to be 10.10.10.6 /24
You set up two hosts, as you have, with 10.10.10.2 and 10.10.10.4 /29 with gateway 10.10.10.6

Now, most of the hosts are in 10.10.10.0 /24 and some of the hosts are in 10.10.10.29 which overlap.
The gateway is the same 10.10.10.6 /24.

Now the 2 odd hosts, broadcast (and listen) on 10.10.10.7; their gateway is at 10.10.10.6.
The other hosts broadcast (and listen) on 10.10.10.255; their gateway is also at 10.10.10.6.
The other hosts can "see" the 2 odd hosts and, I believe, vice versa because:
When a packet is launched from any of the other hosts, it goes out on the wire and the destination device gets it directly.
When a packet is launched from the 2 odd hosts, if it's destined for it's matching host then the packet goes out on the wire and the destination device gets it directly.
When a packet is launched from the 2 odd hosts, if it's destined for any of the other hosts, it goes to the gateway which puts it right back onto the LAN destined for the intended destination.

I'm not suggesting you do this.....

You might also look at the attached.
How-Subnets-Work-in-Practice.pdf
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jskfanAuthor Commented:
I guess Tim Phillips, is closer to what I need to understand,

Once I use 10.10.10.0/30 in point to point, then the only time I can use 10.10.10.0 again, is if I use /31 or as Tim said I can keep /30 but I should go by increment of 4 in the last Octet:
10.10.10.0/30
10.10.10.4/30
10.10.10.8/30

Which means the 10.10.10.X will be used for just point to point, because anything less than /30 for 10.10.10.X will cause overlap.
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Fred MarshallPrincipalCommented:
You can't use /31 as it's not a useful network.

Yes, you can use /30 as I mentioned, up to 64 times in /24.
But you haven't really told us what you *need*.
One subnet.
Two subnets.
20 subnets?
etc.
Having more subnets can present a real chore in terms of routing.  So you should have as few as possible in that regard.

What is it you need to do really?
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jskfanAuthor Commented:
I was just trying to understand the purpose of using /30 in point to point. I know it provides 2 usable host addresses in a specific network...but it does not save the rest of ip addresses unless if you use them in other point to point connections...
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JustInCaseCommented:
It does save them. You can use 10.10.10.8/29 as your next network (6 hosts), and after that you can use 10.10.10.16/28 (14 hosts) 10.10.10.32/27 (30 hosts) 10.10.10.64/26 (62 hosts) etc
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jskfanAuthor Commented:
Thank you Guys for your explanation!!
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Don JohnstonInstructorCommented:
You can't use /31 as it's not a useful network.

Sure it is.  The concept of /31 masks for point-to-point networks has been around for about 15 years.

Definitely an advanced concept since it's not commonly used.

http://tools.ietf.org/html/rfc3021
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Fred MarshallPrincipalCommented:
Thanks for the points.
Don Johnson pointed out something I didn't know re: /31.  
Yet, the concept of "saving" IP addresses eludes me.  
So the notion of /31 seems strange.
For example:
You could have
10.10.10.0 /31 to connect 10.10.10.0 and 10.10.10.1
10.10.10.2/31 to connect 10.10.10.2 and 10.10.10.3
But why not do this:
10.10.10.0 /24 to connect 10.10.10.1 and 10.10.10.2 (or .3, .4 or....)
and
10.10.11.0 /24  to connect 10.10.11.1 and 10.10.11.2 (or .3, .4 or....)?
Well, *unless* one wants to make it complicated.
Obviously, in this situation, the subnets could be smaller (or bigger).

I'm not necessarily advocating this but I ask because it may help illuminate a misunderstanding.
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