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Subnetting

Last response: in Networking
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March 8, 2010 4:38:59 PM

Hello,
Plz tell me that why a mask of /9 and /31 are not used for subnetting

More about : subnetting

March 8, 2010 5:00:23 PM

I don't know anything about /9 not being used (I think it is, I see no reason it shouldn't), but /31 makes no sense since all it reserves is a single IP address for the subnet. And what good is a network that can only host one IP address?

http://www.oav.net/mirrors/cidr.html
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March 9, 2010 3:33:36 AM

Well... sometimes when configuring switches / routers / firewalls (Cisco in particular) you define the range by it's network ID and the mask. For example the below would be used to allow sources 10.10.10.10-10.10.10.11 to destination 10.10.11.1:

access-list example_acl permit ip 10.10.10.10 255.255.255.254 10.10.11.1 255.255.255.255

You may also wish to read up on it's use in point to point networks as described in RFC3021:

http://tools.ietf.org/html/rfc3021
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March 21, 2010 12:51:00 PM

thankz to both of you.. i got my answer
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March 22, 2010 10:35:04 PM

A /9 subnet mask wastes 128 IP addresses because the 128-255 block can't be used since it contains the broadcast address. The /31 or .254 can't be used because you wouldn't have any room for the wire's addresses (network and broadcast). /30 or .252 is the smallest you would use--typically on point to point links.
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