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Further explanation please

Posted on 2004-09-08
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Last Modified: 2010-04-17
I  am hoping someone could explain to me in the following SNMP community string set up, what the
0.0.0.15 after the IP address means.

I was explained this senero:

Further, it's also best practice to restrict the source addresses from where you can make these sorts of transactions.

for instance, on a cisco router you might want something like this:

snmp-server community ThePasswordIsSecret RO 10

access-list 10 permit 192.168.33.128 0.0.0.15

This way only machines addressed within this range that know the community string can grab information about your gear.

As I am learning about community strings, I seem to be getting a little deeper in  other things.
This is a good thing for me. That is why I joined this site. It is well worth the money. And for those of you helping, I really do say prayers to the Lord for this site. But back to my question.

I am familiar with TCP/IP but not from the routing aspect where I am looking at something like this 192.168.33.128 0.0.0.15

Can some one provide an explanation and or a link to a site that can bring me up to speed pretty quick with a basic understanding, as I know that there will be a lot more to learn on this.

Thanks in advance… Michael
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Question by:Linux_Hawk
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PennGwyn earned 200 total points
ID: 12008252
This is a "wildcard mask"; it's sort of the inverse of a subnet mask.

If you write out 0.0.0.15 in binary, you get

00000000.00000000.00000000.00001111

So this mask tells the router that the last four bits of the IP address can have any value.

So "192.168.33.128 0.0.0.15" is access-list-ese for the range 192.168.33.128-192.168.33.143, inclusive.

(This applies to access lists used anywhere, not just in snmp-server config.)

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by:mikebernhardt
mikebernhardt earned 150 total points
ID: 12008612
If you understand subnet masking at all, an easy way to remember what those wild-card masks mean is, subtract each octet from 255. that will give you the subnet mask. So 192.168.33.128 0.0.0.15 would give you the subnet of 192.168.33.128 255.255.255.240, which is the subnet PennGwyn gave you.

Here is a good link that explains subnet masking, with a link to a calculator
http://www.webopedia.com/TERM/S/subnet_mask.html
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by:pedrow
pedrow earned 150 total points
ID: 12011671
also, the inverse bitmask can be easy in that with this:
0.0.0.15

if you start at zero, to 15 is 16. So it's a subnet of 16 addresses. bottom is network number, in this case:
192.168.33.128

top is broadcast, or 128 + 15 = 192.168.33.143

your usable hosts fall between them...
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