Figuring out Subnet mask

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Jan 22nd, 2009
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Hi all,

If you were given an IP address only and want to figure out the subnet mask, how would you do that? Here is an examples: IP address:

Thanks in advance,


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Jon Marshall Thu, 01/22/2009 - 11:46
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It's not possible to say. You can say that the "natural" mask for a 10.x.x.x network is but that doesn't mean that is the subnet mask in use.

With only a single IP address it's not possible to say.


godspeeds Thu, 01/22/2009 - 20:22
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The IP address always comes with a network mask no matter fix or dynamic IP. Can't tell you the exactly subnet mask with a single ip address.

Joseph W. Doherty Fri, 01/23/2009 - 04:30
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Only if the IP addressing was using "classful" addresses from RFC 791 could you know the subnet mask without it being explicit. When it does, the high order network bits will determine the mask. (This also assumes no subnetting.)


JORGE RODRIGUEZ Fri, 01/23/2009 - 04:47
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Perhaps if origical poster could elaborate a bit more in terms of who is giving the IP address would probably help, but plain and simple as others have indicated it is hard to tell the mask by looking at a single IP address alone, think of this, much like a home address without specifying CITY STATE.

skhirbash Fri, 01/23/2009 - 08:07
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Thanks to all for your responses. I inherited a network from a prior engineer and I was in the process of analyzing the network diagrams. The network diagrams don't show the subnet mask but only show the IP address, so that's why I was inquring about a way of finding the subnet mask based on the IP address only.

I hope this helps!

Thanks again,


JORGE RODRIGUEZ Fri, 01/23/2009 - 09:13
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Understandable and I see your point, most of us Im sure usually use in diagrams at least I do the mask in bits represented in its decimal value /23 /24 etc..

but in the case of trying to verify which networks are in placed/advertized is looking at routing tables on companies central CORE router, routing tables would definately provide mask information for all your private networks.



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