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Subnets

Posted on 2007-03-22
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Last Modified: 2008-01-09
I currently have one site in Active Directory and its network is 10.10.x.x 255.255.0.0.   I will be adding 2 more remote sites for a total of 3.   So I need to subnet the 10.10.0.0 network.   I am hoping to do it in a way that has the least amount of disruption to my current scheme.   If I use a 19 bit mask (255.255.224.0) it will give me 8 subnets:  10.10.0.0
                                10.10.32.0
                                10.10.64.0
                                10.10.96.0
                                10.10.128.0 etc..

I would love to use the first 3, 10.10.0.0,.32.0 and .64.0 for my 3 sites.   But every subnet calculator does not include the 10.10.0.0 subnet.   Why is that?   Why can't I use the 10.10.0.0 -10.10.31.254 network?
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Question by:drichard16
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by:rajendraone
ID: 18773426
You can use 10.10.0.0 to 10.10.31.254 without any problem. I don't know what specific subnet calculator you use. Anyway, I do these calculations manually within a minute.
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rajendraone earned 250 total points
ID: 18773600
Also, using subnet calculator I am getting 10.10.0.0/19 network as well. I am using Solaris Advance Subnet Calculator (free download). But I always recommend you doing this sort of calculator manually that will make life lot easier.

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by:The--Captain
The--Captain earned 250 total points
ID: 18778270
>But every subnet calculator does not include the 10.10.0.0 subnet.  
>Why is that?   Why can't I use the 10.10.0.0 -10.10.31.254 network?

If the subnet calculator strictly adheres to the actual RFCs, then 10.10.0.0 might be designated as a reserved subnet - just as the first and last IPs of any subnet associated with an actual interface are reserved according to the RFC, so are the first and last subnets of any according declaration.  No equipment manufactured in approximately the last 8 years really cares if you actually use the first or last subnet, but IIRC the RFC still says that such subnets are technically reserved (although folks use a mask of /25 all the time and it works just fine, even though the RFC says that the first and last subnets are reserved, which eliminates all apparently useable IPs)

I think I found a better explanation:

From: http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080093f18.shtml

"Problems with Subnet Zero and the All-Ones Subnet

Traditionally, it was strongly recommended that subnet zero and the all-ones subnet not be used for addressing. According to RFC 950 leavingcisco.com, "It is useful to preserve and extend the interpretation of these special (network and broadcast) addresses in subnetted networks. This means the values of all zeros and all ones in the subnet field should not be assigned to actual (physical) subnets." This is the reason why network engineers required to calculate the number of subnets obtained by borrowing three bits would calculate 23-2 (6) and not 23 (8). The -2 takes into account that subnet zero and the all-ones subnet are not used traditionally.
Subnet-Zero

Using subnet zero for addressing was discouraged because of the confusion inherent in having a network and a subnet with indistinguishable addresses.

With reference to our example above, consider the IP address 172.16.1.10. If you calculate the subnet address corresponding to this IP address, the answer you arrive at is subnet 172.16.0.0 (subnet zero). Note that this subnet address is identical to network address 172.16.0.0, which was subnetted in the first place, so whenever you perform subnetting, you get a network and a subnet (subnet zero) with indistinguishable addresses. This was formerly a source of great confusion.

Prior to Cisco IOS® Software Release 12.0, Cisco routers, by default, did not allow an IP address belonging to subnet zero to be configured on an interface. However, if a network engineer working with a Cisco IOS software release older than 12.0 finds it safe to use subnet zero, the ip subnet-zero command in the global configuration mode can be used to overcome this restriction. As of Cisco IOS Software Release 12.0, Cisco routers now have ip subnet-zero enabled by default, but if the network engineer feels that it is unsafe to use subnet zero, the no ip subnet-zero command can be used to restrict the use of subnet zero addresses.

In versions prior to Cisco IOS Software Release 8.3, the service subnet-zero command was used."


Hope that helps.

Cheers,
-Jon

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