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Subnet question for private B class 172.16.x.x  ip range

Posted on 2007-03-29
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Last Modified: 2010-09-05
I support a network that is setup a lot like this example: ( http://www.experts-exchange.com/Networking/Misc/Q_21105296.html?sfQueryTermInfo=1+172.16+255.240.0.0+privat )
172.16.1.x for routers
172.16.2.x for switches
172.16.3.x for servers
172.16.4.x for network printers
172.16.5.x for management devices
172.16.6.x for DHCP
172.16.7.x for wireless access points
etc., etc.
with a subnet of 255.255.0.0

After reading RFC 1918 ( http://www.faqs.org/rfcs/rfc1918.html ) that states that a private B class IP should have a network address of 172.16.x.x with a subnet of 255.240.0.0. I can switch my subnet right now from 255.255.0.0 to 255.240.0.0 without much difficulty. After reading the post on Experts Exchange I am a little confused on what is the best practice. Should I have the /12 subnet or is the /16 subnet for the private range known as an acceptable practice?
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Question by:Natldiag
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Expert Comment

by:learn2earn
ID: 18817351
If I am reading your info correctly
you want to know if you should be using a subnet mask of 255.240.0.0 ( answer is no )
You should use subnet mask 255.255.0.0 because you are using a Class B.

Class A: 255.0.0.0
Class B: 255.255.0.0
Class C: 255.255.255.0
http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800a67f5.shtml
I hope this helps!

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Expert Comment

by:learn2earn
ID: 18817377
To seperate your networks you would use a subnet mask of ( 255.255.255.0 ).
With this mask it will break out your networks.
172.16.1.x for routers
172.16.2.x for switches
172.16.3.x for servers
172.16.4.x for network printers
172.16.5.x for management devices
172.16.6.x for DHCP
172.16.7.x for wireless access points
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Author Comment

by:Natldiag
ID: 18817955
learn2earn thanks for your feedback. Currently we do not have it subnetted, so we just have one subnet with the /16 subnet. Looking at RFC 1918 ( http://www.faqs.org/rfcs/rfc1918.html ) , it looks like 172.16.x.x - 172.31.x.x is reserved for a private range? Could you please explain section 3 RFC 1918 to me if it is incorrect that you would use the 255.240.0.0 subnet for a private 172.16.x.x range? I know that normally the  last two octets are subnetted in a class B range, which is why I became curious about this.
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Accepted Solution

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lrmoore earned 250 total points
ID: 18818116
>Could you please explain section 3 RFC 1918 to me if it is incorrect that you would use the 255.240.0.0 subnet for a private 172.16.x.x range?

You are referring to this section of the RFC, and I can understand your confusion..

The Internet Assigned Numbers Authority (IANA) has reserved the
   following three blocks of the IP address space for private internets:
     10.0.0.0        -   10.255.255.255  (10/8 prefix)
     172.16.0.0      -   172.31.255.255  (172.16/12 prefix)
     192.168.0.0     -   192.168.255.255 (192.168/16 prefix)

That simply states that particular block of Class B addresses are reserved for designation as "private internets". The /12 is simply a CIDR notation that defines the summary group.
That does not relinquish the fact that all 172.x.x.x IPs are by definition Class B, and Class B subnet mask is defined 255.255.0.0  /16

There is simply zero value in you changing out your existing /16 mask infrastructure.
Remember, you are free to use any mask between /12 and /30 and any combination thereof using variable length subnet masks (VLSM) in any manner you choose.
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