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What does /16 mean after an IP number mean?

 
 
slate_leeper
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      15-05-2004, 12:12 AM
When IP addresses are listed, what do numbers after a slash following
an IP number mean, such as 158.113.0.0/16 or 161.88.240.0/20?

-dan z-



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data64
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      15-05-2004, 01:11 AM
slate_leeper <(E-Mail Removed) > wrote in
news:(E-Mail Removed):

> When IP addresses are listed, what do numbers after a slash following
> an IP number mean, such as 158.113.0.0/16 or 161.88.240.0/20?
>
> -dan z-


That would be the subnet mask.
There is a tutorial on that
http://compnetworking.about.com/libr.../aa043000a.htm

Definitions from wikipedia and webopedia
http://en.wikipedia.org/wiki/Netmask
http://www.webopedia.com/TERM/S/subnet_mask.html

data64
 
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Jay T. Blocksom
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      28-05-2004, 08:39 PM
On Fri, 14 May 2004 20:12:21 -0400, in <alt.privacy.spyware>, slate_leeper
<(E-Mail Removed) > wrote:
>
> When IP addresses are listed, what do numbers after a slash following
> an IP number mean, such as 158.113.0.0/16 or 161.88.240.0/20?
>

[snip]

You're referring to CIDR ("Classless Inter-Domain Routing") notation. The
number after the "/" identifies the number of significant bits used to
identify a particular range of IP addresses. In *some* cases, those ranges
happen to also belong to a particular network (which was the original
intention of CIDR).

Taking the examples you cited...

"158.113.0.0/16" can also be expressed as "[158.113.0.0 - 158.113.255.255],
and represents a block of 64K addresses. Coincidentally, this particular
example is also a classic "Class B" block (owned by UNICEF).

"161.88.240.0/20" can also be expressed as "[161.88.240.0 - 161.88.255.255]"
and represents a block of 4K addresses. This too happens to be an actual
IANA/RIPE-delegated (to Philips Electronics North America) block; but it does
*not* fall on traditional "Class A/B/C" boundaries. Hence, without CIDR, it
would have been difficult/impossible to define/implement this particular
network.

--

Jay T. Blocksom
--------------------------------
Appropriate Technology, Inc.
usenet01[at]appropriate-tech.net


"They that can give up essential liberty to obtain a little temporary
safety deserve neither liberty nor safety."
-- Benjamin Franklin, Historical Review of Pennsylvania, 1759.

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Jay T. Blocksom
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      28-05-2004, 08:39 PM
On 15 May 2004 01:11:18 GMT, in <alt.privacy.spyware>, data64 <(E-Mail Removed)>
wrote:
>
> slate_leeper <(E-Mail Removed) > wrote in
> news:(E-Mail Removed):
>
> > When IP addresses are listed, what do numbers after a slash following
> > an IP number mean, such as 158.113.0.0/16 or 161.88.240.0/20?
> >
> > -dan z-

>
> That would be the subnet mask.

[snip]

Well, sometimes. But in practice, CIDR notation can be used to express *ANY*
"subnettable" (if you will) block of addresses, regardless of whether any such
network exists or not.

--

Jay T. Blocksom
--------------------------------
Appropriate Technology, Inc.
usenet01[at]appropriate-tech.net


"They that can give up essential liberty to obtain a little temporary
safety deserve neither liberty nor safety."
-- Benjamin Franklin, Historical Review of Pennsylvania, 1759.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Unsolicited advertising sent to this E-Mail address is expressly prohibited
under USC Title 47, Section 227. Violators are subject to charge of up to
$1,500 per incident or treble actual costs, whichever is greater.
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slate_leeper
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      29-05-2004, 02:51 PM
Thanks for the reply, Jay. I'm still a little confused. If the number
after "/" is the number of significant bits, why does /16 represent
64K addresses, while /20 represents only 4K?

-dan z-

Note: email address used is real, but currently turned off due to
large quantities of virus/worm email being received. Please reply to
group.


On Fri, 28 May 2004 16:39:25 -0400, Jay T. Blocksom
<(E-Mail Removed)> wrote:

>On Fri, 14 May 2004 20:12:21 -0400, in <alt.privacy.spyware>, slate_leeper
><(E-Mail Removed) > wrote:
> >
> > When IP addresses are listed, what do numbers after a slash following
> > an IP number mean, such as 158.113.0.0/16 or 161.88.240.0/20?
> >

> [snip]
>
>You're referring to CIDR ("Classless Inter-Domain Routing") notation. The
>number after the "/" identifies the number of significant bits used to
>identify a particular range of IP addresses. In *some* cases, those ranges
>happen to also belong to a particular network (which was the original
>intention of CIDR).
>
>Taking the examples you cited...
>
>"158.113.0.0/16" can also be expressed as "[158.113.0.0 - 158.113.255.255],
>and represents a block of 64K addresses. Coincidentally, this particular
>example is also a classic "Class B" block (owned by UNICEF).
>
>"161.88.240.0/20" can also be expressed as "[161.88.240.0 - 161.88.255.255]"
>and represents a block of 4K addresses. This too happens to be an actual
>IANA/RIPE-delegated (to Philips Electronics North America) block; but it does
>*not* fall on traditional "Class A/B/C" boundaries. Hence, without CIDR, it
>would have been difficult/impossible to define/implement this particular
>network.


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radioman
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      30-05-2004, 03:06 PM
Hi, you need to count from the left most bit the number after the/
so as an example:
192.168.1.0/24 means 192.168.1.xxx i.e. xxx can be 0 to 15.
192.168.0.0/16 means 192.168.xxx.xxx i.e. can be 0 to 254.

Remember that the dotted decimal is just a representation of the binary
number.
For example 11111111.11111111.11111111.00000000 is the same as
255.255.255.0 and 255.255.255.0/24, count the ones above


So the number after the/ means the number of bits counted from the left
(most significant bit) in binary. Hope that makes sense.

--
--


radioman

Please remove "spamnot" to reply.

http://homepage.ntlworld.com/daves.den



"slate_leeper" <(E-Mail Removed) > wrote in message
news:(E-Mail Removed)...
> Thanks for the reply, Jay. I'm still a little confused. If the number
> after "/" is the number of significant bits, why does /16 represent
> 64K addresses, while /20 represents only 4K?
>
> -dan z-
>
> Note: email address used is real, but currently turned off due to
> large quantities of virus/worm email being received. Please reply to
> group.
>
>
> On Fri, 28 May 2004 16:39:25 -0400, Jay T. Blocksom
> <(E-Mail Removed)> wrote:
>
> >On Fri, 14 May 2004 20:12:21 -0400, in <alt.privacy.spyware>,

slate_leeper
> ><(E-Mail Removed) > wrote:
> > >
> > > When IP addresses are listed, what do numbers after a slash following
> > > an IP number mean, such as 158.113.0.0/16 or 161.88.240.0/20?
> > >

> > [snip]
> >
> >You're referring to CIDR ("Classless Inter-Domain Routing") notation.

The
> >number after the "/" identifies the number of significant bits used to
> >identify a particular range of IP addresses. In *some* cases, those

ranges
> >happen to also belong to a particular network (which was the original
> >intention of CIDR).
> >
> >Taking the examples you cited...
> >
> >"158.113.0.0/16" can also be expressed as "[158.113.0.0 -

158.113.255.255],
> >and represents a block of 64K addresses. Coincidentally, this particular
> >example is also a classic "Class B" block (owned by UNICEF).
> >
> >"161.88.240.0/20" can also be expressed as "[161.88.240.0 -

161.88.255.255]"
> >and represents a block of 4K addresses. This too happens to be an actual
> >IANA/RIPE-delegated (to Philips Electronics North America) block; but it

does
> >*not* fall on traditional "Class A/B/C" boundaries. Hence, without CIDR,

it
> >would have been difficult/impossible to define/implement this particular
> >network.

>
> - -
> Protect your civil rights!
> Let the politicians know how you feel.
> Join or donate to the NRA today!
> http://membership.nrahq.org/default....ignid=XR014887



 
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Jay T. Blocksom
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      31-05-2004, 12:00 PM
On Sat, 29 May 2004 10:51:00 -0400, in <alt.privacy.spyware>, slate_leeper
<(E-Mail Removed) > wrote:
>
> Thanks for the reply, Jay. I'm still a little confused. If the number
> after "/" is the number of significant bits, why does /16 represent
> 64K addresses, while /20 represents only 4K?
>

[snip]

Because those bits don't represent the *size* of the network; but rather, the
base address of the IP range in question. For a somewhat simpler example, the
entire Genuity network, comprising more than 16 million possible IP addresses,
can be represented as "4.0.0.0/8", because only 8 bits are needed to define
that first quad; all the other bits (i.e., the remaining three quads) are
irrelevant, because they are *part* of the defined IP block (i.e., the Genuity
network).

--

Jay T. Blocksom
--------------------------------
Appropriate Technology, Inc.
usenet01[at]appropriate-tech.net


"They that can give up essential liberty to obtain a little temporary
safety deserve neither liberty nor safety."
-- Benjamin Franklin, Historical Review of Pennsylvania, 1759.

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under USC Title 47, Section 227. Violators are subject to charge of up to
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slate_leeper
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      31-05-2004, 02:47 PM
OK, I think I understand now. Thanks to both you and Jay.

-dan-




On Sun, 30 May 2004 16:06:33 +0100, "radioman"
<(E-Mail Removed)> wrote:

>Hi, you need to count from the left most bit the number after the/
>so as an example:
>192.168.1.0/24 means 192.168.1.xxx i.e. xxx can be 0 to 15.
>192.168.0.0/16 means 192.168.xxx.xxx i.e. can be 0 to 254.
>
>Remember that the dotted decimal is just a representation of the binary
>number.
>For example 11111111.11111111.11111111.00000000 is the same as
>255.255.255.0 and 255.255.255.0/24, count the ones above
>
>
>So the number after the/ means the number of bits counted from the left
>(most significant bit) in binary. Hope that makes sense.
>
>--


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Let the politicians know how you feel.
Join or donate to the NRA today!
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Ian
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      31-05-2004, 03:29 PM
In message <(E-Mail Removed)>, slate_leeper
<(E-Mail Removed)> writes
>Thanks for the reply, Jay. I'm still a little confused. If the number
>after "/" is the number of significant bits, why does /16 represent
>64K addresses, while /20 represents only 4K?
>


IP addresses consist of 32 bits (for the addresses that most people use
today). Out of those 32 bits, the first n bits (where n is the number
after the slash) represent the network address, and the remaining bits
represent the address of a host on that network.

For a /16 network, the first 16 bits are the network address, and the
remaining 16 bits represent a particular host. Those remaining 16 bits
can represent up to 64K separate host addresses (i.e. 2 raised to the
power 16).

For a /20 network, the first 20 bits are the network address, leaving
just 12 bits (i.e. 32-20) for the host address. Those 12 bits can
represent 4K host addresses (i.e. 2 raised to the power 12).

So as the number /n increases, the network address becomes longer,
leaving fewer bits available to specify hosts on that network.

Hope this helps.

--
Ian
 
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