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subnet

Posted on 2013-05-14
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Last Modified: 2013-05-31
HI there
sorry guys this may be a stupid question but i would like to ask about subneting .Currently im working  with ip address 10.10.8.0/21 which was given to  me by service provider , Im further divining this network into smaller chunks  and assigning this to interf vlans on cisco 4500 switch .Company im working currently for like to keep first 8  addresses as reserve and 10.10.8.2 of this first subnet  will be internet connection to outside world through provider network and 10.10.8.1 address .Now i can subnet this into 4/26 but i will loose quite few  addresses from first 64 subnet .I try to resubnet then like 10.10.8.0/29 then 10.10.8.16/26 and 10.10.8.80/26 but that will result in subnet clashes so the question is can i somehow resubnet this so i reserve first 8 addresses and then resubnet rest of the subnet with out clash scenario or is it impossible to do such task . Hope you get me ;-)) thanks guys
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Question by:homolama
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by:Don Johnston
ID: 39164348
You say that 10.10.8.1 and 10.10.8.2 are used for the internet connection. What is the mask on this link?
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by:asavener
ID: 39164499
You can subnet 10.10.8.0/21 gives you a range of addresses from 10.10.8.0 through 10.10.15.255.

If you create a subnet 10.10.8.0.0/29, then you can create 10.10.8.8/29, 10.10.8.16/28, 10.10.8.32/27, 10.10.8.64/26, 10.10.8.128/25.
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by:Fred Marshall
ID: 39165137
You say the company wants to keep the first 8 addresses in reserve.  OK but is that "per subnet" or "per site"?  That makes a big difference!

Not really knowing the intent, but under the assumption that it is only the first subnet needed the reserved 8 addresses (seems rather low) then I would do this:

1) Consider NAT and that you might have lots more addresses and subnet possibilities. You surely aren't stuck with a private address range "given" by the ISP.  It's just an accident.

and:

2) Subnetting and reserving addresses aren't the same thing.  It's not clear to me that reserving addresses AND putting them into a separate subnet is particularly useful.  But you may have a reason.

.. but sticking with that for now:

1) Split out the high 1/2 of 10.10.8.0/21 which would be:
10.10.12.0/22 for 1022 addresses

2) Split out the high 1/2 of 10.10.8.0/22 which would be:
10.10.9.0/23 for 510 addresses

3) Split out the high 1/2 of 10.10.8.0/23 which would be:
10.10.98.0/24 for 254 addresses

4) Split out the high 1/2 of 10.10.8.0/24 which would be:
10.10.8.128/25 for 126 addresses

5) Split out the high 1/2 of 10.10.8.0/25 which would be:
10.10.8.64/26 for 62 addresses

6) Split out the high 1/2 of 10.10.8.0/26 which would be:
10.10.8.32/27 for 30 addresses

7) Split out the high 1/2 of 10.10.8.0/27 which would be:
10.10.8.16/28 for 14 addresses

8) Split out the high 1/2 of 10.10.8.0/28 which would be:
10.10.8.8.8/29 for 6 addresses.

So, as you can see, there's a point of diminishing returns and you could stop and "throw away" the last few of these subnets OR perhaps better, don't split out 8 at all but, rather, split out the remaining addresses like 10.10.8.32/27 for example.  Then the addresses aren't precluded but are just reserved for margin.

Of course, either way you do it, having an 8 address subnet forces one to split the subnets up as shown above.  That's a bit restrictive and I don't like the idea of so many subnets.  But, I believe this is the best one can do given the assumptions.  If you put the 8 addresses at the top, the same thing happens.

And, of course, a subnet of "8 addresses" only provides for 6 hosts.  I rather neglected that detail above except for the case where the bottom subnet is larger anyway.
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by:asavener
ID: 39165569
I think you may also want to get clarification on what "in reserve" means.  I could be that they just don't want those addresses to be assigned to devices.
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Author Comment

by:homolama
ID: 39171289
ok sorry guys basically maybe i been not so clear , sorry my English is bit of wreckage.Any way to simplify whole matter the question is can i subnet 10.10.0.0 255.255.255.0

 to 10.10.0.1 255.255.255.248
then next on  10.10.0.8 255.255.255.192
next on  10.10.0.72 255.255.255.192and so on
without clashing between addressing

 i know i can do it this way but the company insist that first 10.10.0.1 and 2 will be use between routers so i do not know how to save addresses i can use /26 but i will be left with 60 unused  addresses
10.10.0.0 255.255.255.192
10.10.0.64 255.255.255.192
10.10.0.128 255.255.255.192
10.10.0.192 255.255.255.224
10.10.0.224 255.255.255.240
10.10.0.240 255.255.255.240

hope this is bit more clearer or that you get me ;-)) thanks very much for all
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by:Don Johnston
ID: 39171320
It's all going to depend on what mask you have to use on the 10.10.0.1 and 10.10.0.2 hosts.

If they specify a /24 mask, then you can't subnet it.

However, if you can use a longer mask, then you have some options.

But it all depends on what mask they require on that link.
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Accepted Solution

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asavener earned 500 total points
ID: 39171344
to 10.10.0.1 255.255.255.248
then next on  10.10.0.8 255.255.255.192
next on  10.10.0.72 255.255.255.192and so on
without clashing between addressing
Short answer:  no.

If 10.10.0.0 is 255.255.255.248 then 10.10.0.8 has to be 255.255.255.248 or 255.255.255.252.

Then, 10.10.0.16 can be 255.255.255.248 (which forces 10.10.0.24 to be 255.255.255.248) or you can have 10.10.0.16 as 255.255.255.240.

Basically, extending the subnet mask can force an adjacent subnet to use the same subnet mask.
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by:asavener
ID: 39171354
i know i can do it this way but the company insist that first 10.10.0.1 and 2 will be use between routers
OK.  This is different from subnetting the address space, though.  In this case, 10.10.0.1 and 10.10.0.2 are reserved addresses that you cannot use for other devices.  They should still be in the same subnet as your regular devices, though, because the regular devices have to be able to reach the routers.

If you are setting up DHCP, then you would create your DHCP scope, and then exclude these addresses from being handed out to devices.
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by:Fred Marshall
ID: 39171789
You didn't really answer my question about "reserved" addresses vs. "subnets":
You say the company wants to keep the first 8 addresses in reserve.  OK but is that "per subnet" or "per site"?  That makes a big difference!

If I might recommend:
Respond back with a higher-level view of the network(s).  
- how many sites?
- how many computers and other devices?
- how many actual separate subnets do you want or need?
With this information we ought to be able to help better.

Just jumping into subnetting questions without knowing the layout or the objectives is going to be confusing.
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