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| Author |
Ok, what is a subnet in the CCNA
|
|
| Mark Smythe 2002-08-24, 9:42 am |
| Hi All,
With IP address 172.16.10.2 255.255.255.128 the pretend test I took
says there are 9 subnet bits in this address. How could this be? It
looks like 25 bits. Are they insisting this based on Class B? They
dont count the first 16 bits as part of a subnet?
Mark
| |
| Seth Corduan 2002-08-24, 9:42 am |
| I'm by no means an expert, but I feel like taking a crack at this:
The IP number is in the 128-191 range, so "technically" that should make it
a class B. (Near as I can tell, the CCNA is all about what something
"technically" is.) That would take the first 16 right out. (Again, I think)
and that leaves the 8 for the 3rd 255 and of course the 1 for the 128.
In short, yes, because the IP is within the Class B range.
Corrections? Millimiter, this seems up your alley.
Seth C
----- Original Message -----
From: "Mark Smythe" <smythe132@attbi.com>
Newsgroups: alt.certification.cisco
Sent: Saturday, August 24, 2002 9:03 AM
Subject: Ok, what is a subnet in the CCNA
> Hi All,
> With IP address 172.16.10.2 255.255.255.128 the pretend test I took
> says there are 9 subnet bits in this address. How could this be? It
> looks like 25 bits. Are they insisting this based on Class B? They
> dont count the first 16 bits as part of a subnet?
>
> Mark
>
| |
| Seth Corduan 2002-08-24, 10:42 am |
| I'm by no means an expert, but I feel like taking a crack at this:
The IP number is in the 128-191 range, so "technically" that should make it
a class B. (Near as I can tell, the CCNA is all about what something
"technically" is.) That would take the first 16 right out. (Again, I think)
and that leaves the 8 for the 3rd 255 and of course the 1 for the 128.
In short, yes, because the IP is within the Class B range.
Corrections? Millimiter, this seems up your alley.
Seth C
----- Original Message -----
From: "Mark Smythe" <smythe132@attbi.com>
Newsgroups: alt.certification.cisco
Sent: Saturday, August 24, 2002 9:03 AM
Subject: Ok, what is a subnet in the CCNA
> Hi All,
> With IP address 172.16.10.2 255.255.255.128 the pretend test I took
> says there are 9 subnet bits in this address. How could this be? It
> looks like 25 bits. Are they insisting this based on Class B? They
> dont count the first 16 bits as part of a subnet?
>
> Mark
>
| |
|
| Ok, my turn :
25 bits is correct if you refer to the entire mask, since we have 16 bits
for the class B default mask + 8 in the third octet + the first bit in the
fourth octet = 25
However, the subnet mask is 9 bits that have been "borrowed" from the host
part of the IP address.
default mask for the class + subnet bits borrowed + host portion bits = 32
in this case it would be like this :
16 + 9 + 7 = 32
Technically the subnet mask is the portion of bits borrowed from the host
portion.
So, in other words, that is a /25 class B address, with a subnet mask of 9
bits borrowed from the host part.
hth.
Robi.
"Seth Corduan" <network@kalach.freehosting.net> wrote in message
news:j9N99.167714$2p2.7453773@bin4.nnrp.aus1.giganews.com...
> I'm by no means an expert, but I feel like taking a crack at this:
>
> The IP number is in the 128-191 range, so "technically" that should make
it
> a class B. (Near as I can tell, the CCNA is all about what something
> "technically" is.) That would take the first 16 right out. (Again, I
think)
> and that leaves the 8 for the 3rd 255 and of course the 1 for the 128.
>
> In short, yes, because the IP is within the Class B range.
>
>
> Corrections? Millimiter, this seems up your alley.
>
> Seth C
>
> ----- Original Message -----
> From: "Mark Smythe" <smythe132@attbi.com>
> Newsgroups: alt.certification.cisco
> Sent: Saturday, August 24, 2002 9:03 AM
> Subject: Ok, what is a subnet in the CCNA
>
>
> > Hi All,
> > With IP address 172.16.10.2 255.255.255.128 the pretend test I took
> > says there are 9 subnet bits in this address. How could this be? It
> > looks like 25 bits. Are they insisting this based on Class B? They
> > dont count the first 16 bits as part of a subnet?
> >
> > Mark
> >
>
>
>
| |
| Mark Smythe 2002-08-24, 10:42 am |
|
Seth Corduan wrote:
> I'm by no means an expert, but I feel like taking a crack at this:
>
> The IP number is in the 128-191 range, so "technically" that should make it
> a class B. (Near as I can tell, the CCNA is all about what something
> "technically" is.) That would take the first 16 right out. (Again, I think)
> and that leaves the 8 for the 3rd 255 and of course the 1 for the 128.
But technically the first 16 bits are part of the subnet. I dont understand how
being a class B address would eliminate the need to consider those bits i.e.
172.16.10.1 is different subnet than 172.17.10.1 if both are Class B . But
the logic of this test would say that there are no subnet bits. Or maybe they
try to differentiate between
a "network" address or a "subnet" of that network address. Or maybe I have the
terminolgy wrong or something. Although I have studied IP and the classes, in
reality I have never thought of classes when setting up IP schemes. To me the
number of network bits starts from the far left . I could also have
172.16.10.1 224.0.0.0. Would a router actually not let me configure an
address if it is out of class? I appreciate all the help.
Mark
>
>
> In short, yes, because the IP is within the Class B range.
>
> Corrections? Millimiter, this seems up your alley.
>
> Seth C
>
> ----- Original Message -----
> From: "Mark Smythe" <smythe132@attbi.com>
> Newsgroups: alt.certification.cisco
> Sent: Saturday, August 24, 2002 9:03 AM
> Subject: Ok, what is a subnet in the CCNA
>
> > Hi All,
> > With IP address 172.16.10.2 255.255.255.128 the pretend test I took
> > says there are 9 subnet bits in this address. How could this be? It
> > looks like 25 bits. Are they insisting this based on Class B? They
> > dont count the first 16 bits as part of a subnet?
> >
> > Mark
> >
| |
|
| Ok, my turn :
25 bits is correct if you refer to the entire mask, since we have 16 bits
for the class B default mask + 8 in the third octet + the first bit in the
fourth octet = 25
However, the subnet mask is 9 bits that have been "borrowed" from the host
part of the IP address.
default mask for the class + subnet bits borrowed + host portion bits = 32
in this case it would be like this :
16 + 9 + 7 = 32
Technically the subnet mask is the portion of bits borrowed from the host
portion.
So, in other words, that is a /25 class B address, with a subnet mask of 9
bits borrowed from the host part.
hth.
Robi.
"Seth Corduan" <network@kalach.freehosting.net> wrote in message
news:j9N99.167714$2p2.7453773@bin4.nnrp.aus1.giganews.com...
> I'm by no means an expert, but I feel like taking a crack at this:
>
> The IP number is in the 128-191 range, so "technically" that should make
it
> a class B. (Near as I can tell, the CCNA is all about what something
> "technically" is.) That would take the first 16 right out. (Again, I
think)
> and that leaves the 8 for the 3rd 255 and of course the 1 for the 128.
>
> In short, yes, because the IP is within the Class B range.
>
>
> Corrections? Millimiter, this seems up your alley.
>
> Seth C
>
> ----- Original Message -----
> From: "Mark Smythe" <smythe132@attbi.com>
> Newsgroups: alt.certification.cisco
> Sent: Saturday, August 24, 2002 9:03 AM
> Subject: Ok, what is a subnet in the CCNA
>
>
> > Hi All,
> > With IP address 172.16.10.2 255.255.255.128 the pretend test I took
> > says there are 9 subnet bits in this address. How could this be? It
> > looks like 25 bits. Are they insisting this based on Class B? They
> > dont count the first 16 bits as part of a subnet?
> >
> > Mark
> >
>
>
>
| |
| Mark Smythe 2002-08-24, 11:42 am |
|
Seth Corduan wrote:
> I'm by no means an expert, but I feel like taking a crack at this:
>
> The IP number is in the 128-191 range, so "technically" that should make it
> a class B. (Near as I can tell, the CCNA is all about what something
> "technically" is.) That would take the first 16 right out. (Again, I think)
> and that leaves the 8 for the 3rd 255 and of course the 1 for the 128.
But technically the first 16 bits are part of the subnet. I dont understand how
being a class B address would eliminate the need to consider those bits i.e.
172.16.10.1 is different subnet than 172.17.10.1 if both are Class B . But
the logic of this test would say that there are no subnet bits. Or maybe they
try to differentiate between
a "network" address or a "subnet" of that network address. Or maybe I have the
terminolgy wrong or something. Although I have studied IP and the classes, in
reality I have never thought of classes when setting up IP schemes. To me the
number of network bits starts from the far left . I could also have
172.16.10.1 224.0.0.0. Would a router actually not let me configure an
address if it is out of class? I appreciate all the help.
Mark
>
>
> In short, yes, because the IP is within the Class B range.
>
> Corrections? Millimiter, this seems up your alley.
>
> Seth C
>
> ----- Original Message -----
> From: "Mark Smythe" <smythe132@attbi.com>
> Newsgroups: alt.certification.cisco
> Sent: Saturday, August 24, 2002 9:03 AM
> Subject: Ok, what is a subnet in the CCNA
>
> > Hi All,
> > With IP address 172.16.10.2 255.255.255.128 the pretend test I took
> > says there are 9 subnet bits in this address. How could this be? It
> > looks like 25 bits. Are they insisting this based on Class B? They
> > dont count the first 16 bits as part of a subnet?
> >
> > Mark
> >
| |
| Mark Smythe 2002-08-24, 11:42 am |
| Thanks Robi, this is very clear now.
Robi wrote:
> Ok, my turn :
> 25 bits is correct if you refer to the entire mask, since we have 16 bits
> for the class B default mask + 8 in the third octet + the first bit in the
> fourth octet = 25
> However, the subnet mask is 9 bits that have been "borrowed" from the host
> part of the IP address.
>
> default mask for the class + subnet bits borrowed + host portion bits = 32
> in this case it would be like this :
> 16 + 9 + 7 = 32
> Technically the subnet mask is the portion of bits borrowed from the host
> portion.
> So, in other words, that is a /25 class B address, with a subnet mask of 9
> bits borrowed from the host part.
> hth.
> Robi.
>
> "Seth Corduan" <network@kalach.freehosting.net> wrote in message
> news:j9N99.167714$2p2.7453773@bin4.nnrp.aus1.giganews.com...
> > I'm by no means an expert, but I feel like taking a crack at this:
> >
> > The IP number is in the 128-191 range, so "technically" that should make
> it
> > a class B. (Near as I can tell, the CCNA is all about what something
> > "technically" is.) That would take the first 16 right out. (Again, I
> think)
> > and that leaves the 8 for the 3rd 255 and of course the 1 for the 128.
> >
> > In short, yes, because the IP is within the Class B range.
> >
> >
> > Corrections? Millimiter, this seems up your alley.
> >
> > Seth C
> >
> > ----- Original Message -----
> > From: "Mark Smythe" <smythe132@attbi.com>
> > Newsgroups: alt.certification.cisco
> > Sent: Saturday, August 24, 2002 9:03 AM
> > Subject: Ok, what is a subnet in the CCNA
> >
> >
> > > Hi All,
> > > With IP address 172.16.10.2 255.255.255.128 the pretend test I took
> > > says there are 9 subnet bits in this address. How could this be? It
> > > looks like 25 bits. Are they insisting this based on Class B? They
> > > dont count the first 16 bits as part of a subnet?
> > >
> > > Mark
> > >
> >
> >
> >
| |
| steve harris 2002-08-24, 11:42 am |
| Mark Smythe wrote:
>
>
> Seth Corduan wrote:
>
>> I'm by no means an expert, but I feel like taking a crack at this:
>>
>> The IP number is in the 128-191 range, so "technically" that should make
>> it a class B. (Near as I can tell, the CCNA is all about what something
>> "technically" is.) That would take the first 16 right out. (Again, I
>> think) and that leaves the 8 for the 3rd 255 and of course the 1 for the
>> 128.
>
> But technically the first 16 bits are part of the subnet. I dont
> understand how being a class B address would eliminate the need to
> consider those bits i.e.
> 172.16.10.1 is different subnet than 172.17.10.1 if both are Class B .
> But the logic of this test would say that there are no subnet bits. Or
> maybe they try to differentiate between
> a "network" address or a "subnet" of that network address. Or maybe I have
> the terminolgy wrong or something. Although I have studied IP and the
> classes, in reality I have never thought of classes when setting up IP
> schemes. To me the number of network bits starts from the far left . I
> could also have
> 172.16.10.1 224.0.0.0. Would a router actually not let me configure an
> address if it is out of class? I appreciate all the help.
>
> Mark
>
>
>
>>
>>
>> In short, yes, because the IP is within the Class B range.
>>
>> Corrections? Millimiter, this seems up your alley.
>>
>> Seth C
>>
>> ----- Original Message -----
>> From: "Mark Smythe" <smythe132@attbi.com>
>> Newsgroups: alt.certification.cisco
>> Sent: Saturday, August 24, 2002 9:03 AM
>> Subject: Ok, what is a subnet in the CCNA
>>
>> > Hi All,
>> > With IP address 172.16.10.2 255.255.255.128 the pretend test I took
>> > says there are 9 subnet bits in this address. How could this be? It
>> > looks like 25 bits. Are they insisting this based on Class B? They
>> > dont count the first 16 bits as part of a subnet?
>> >
>> > Mark
>> >
Mark,
the short answer is it's the way Cisco does things.
Like if you are using a class B address, it assumes 16 bits already on the
mask. You enter only enter the additional number of bits. It's the way the
Cisco routers are set up.
Wait until you set up OSPF on a Cisco router, you have to put in the mask
backwards. "0.0.255.255"
Steve
| |
| Mark Smythe 2002-08-24, 12:42 pm |
| Thanks Robi, this is very clear now.
Robi wrote:
> Ok, my turn :
> 25 bits is correct if you refer to the entire mask, since we have 16 bits
> for the class B default mask + 8 in the third octet + the first bit in the
> fourth octet = 25
> However, the subnet mask is 9 bits that have been "borrowed" from the host
> part of the IP address.
>
> default mask for the class + subnet bits borrowed + host portion bits = 32
> in this case it would be like this :
> 16 + 9 + 7 = 32
> Technically the subnet mask is the portion of bits borrowed from the host
> portion.
> So, in other words, that is a /25 class B address, with a subnet mask of 9
> bits borrowed from the host part.
> hth.
> Robi.
>
> "Seth Corduan" <network@kalach.freehosting.net> wrote in message
> news:j9N99.167714$2p2.7453773@bin4.nnrp.aus1.giganews.com...
> > I'm by no means an expert, but I feel like taking a crack at this:
> >
> > The IP number is in the 128-191 range, so "technically" that should make
> it
> > a class B. (Near as I can tell, the CCNA is all about what something
> > "technically" is.) That would take the first 16 right out. (Again, I
> think)
> > and that leaves the 8 for the 3rd 255 and of course the 1 for the 128.
> >
> > In short, yes, because the IP is within the Class B range.
> >
> >
> > Corrections? Millimiter, this seems up your alley.
> >
> > Seth C
> >
> > ----- Original Message -----
> > From: "Mark Smythe" <smythe132@attbi.com>
> > Newsgroups: alt.certification.cisco
> > Sent: Saturday, August 24, 2002 9:03 AM
> > Subject: Ok, what is a subnet in the CCNA
> >
> >
> > > Hi All,
> > > With IP address 172.16.10.2 255.255.255.128 the pretend test I took
> > > says there are 9 subnet bits in this address. How could this be? It
> > > looks like 25 bits. Are they insisting this based on Class B? They
> > > dont count the first 16 bits as part of a subnet?
> > >
> > > Mark
> > >
> >
> >
> >
| |
| steve harris 2002-08-24, 12:42 pm |
| Mark Smythe wrote:
>
>
> Seth Corduan wrote:
>
>> I'm by no means an expert, but I feel like taking a crack at this:
>>
>> The IP number is in the 128-191 range, so "technically" that should make
>> it a class B. (Near as I can tell, the CCNA is all about what something
>> "technically" is.) That would take the first 16 right out. (Again, I
>> think) and that leaves the 8 for the 3rd 255 and of course the 1 for the
>> 128.
>
> But technically the first 16 bits are part of the subnet. I dont
> understand how being a class B address would eliminate the need to
> consider those bits i.e.
> 172.16.10.1 is different subnet than 172.17.10.1 if both are Class B .
> But the logic of this test would say that there are no subnet bits. Or
> maybe they try to differentiate between
> a "network" address or a "subnet" of that network address. Or maybe I have
> the terminolgy wrong or something. Although I have studied IP and the
> classes, in reality I have never thought of classes when setting up IP
> schemes. To me the number of network bits starts from the far left . I
> could also have
> 172.16.10.1 224.0.0.0. Would a router actually not let me configure an
> address if it is out of class? I appreciate all the help.
>
> Mark
>
>
>
>>
>>
>> In short, yes, because the IP is within the Class B range.
>>
>> Corrections? Millimiter, this seems up your alley.
>>
>> Seth C
>>
>> ----- Original Message -----
>> From: "Mark Smythe" <smythe132@attbi.com>
>> Newsgroups: alt.certification.cisco
>> Sent: Saturday, August 24, 2002 9:03 AM
>> Subject: Ok, what is a subnet in the CCNA
>>
>> > Hi All,
>> > With IP address 172.16.10.2 255.255.255.128 the pretend test I took
>> > says there are 9 subnet bits in this address. How could this be? It
>> > looks like 25 bits. Are they insisting this based on Class B? They
>> > dont count the first 16 bits as part of a subnet?
>> >
>> > Mark
>> >
Mark,
the short answer is it's the way Cisco does things.
Like if you are using a class B address, it assumes 16 bits already on the
mask. You enter only enter the additional number of bits. It's the way the
Cisco routers are set up.
Wait until you set up OSPF on a Cisco router, you have to put in the mask
backwards. "0.0.255.255"
Steve
| |
| Mark Smythe 2002-08-24, 1:42 pm |
|
steve harris wrote:
> Like if you are using a class B address, it assumes 16 bits already on the
> mask. You enter only enter the additional number of bits. It's the way the
> Cisco routers are set up.
>
Yes, I remember sometime ago setting up a router and having to remember to enter
8 for that setup question so I could have a 255.255.255.0 mask for a 172.x.x.x
ip address. It is all coming clear . Thanks. I shouldnt have any problem on
those types of questions now. I'll just have to remember what they want .
Mark
>
> Wait until you set up OSPF on a Cisco router, you have to put in the mask
> backwards. "0.0.255.255"
>
> Steve
| |
| Mark Smythe 2002-08-24, 2:42 pm |
|
steve harris wrote:
> Like if you are using a class B address, it assumes 16 bits already on the
> mask. You enter only enter the additional number of bits. It's the way the
> Cisco routers are set up.
>
Yes, I remember sometime ago setting up a router and having to remember to enter
8 for that setup question so I could have a 255.255.255.0 mask for a 172.x.x.x
ip address. It is all coming clear . Thanks. I shouldnt have any problem on
those types of questions now. I'll just have to remember what they want .
Mark
>
> Wait until you set up OSPF on a Cisco router, you have to put in the mask
> backwards. "0.0.255.255"
>
> Steve
| |
| Kevin W. 2002-08-24, 8:28 pm |
| Hello,
The first 16 bits are considered the network mask. Any bits borrowed
from the host address are part of the subnet mask. But the CIDR
notation for the address would be 172.16.10.2/25.
Kevin
Mark Smythe <smythe132@attbi.com> wrote in message news:<3D67930D.8EE61571@attbi.com>...
> Hi All,
> With IP address 172.16.10.2 255.255.255.128 the pretend test I took
> says there are 9 subnet bits in this address. How could this be? It
> looks like 25 bits. Are they insisting this based on Class B? They
> dont count the first 16 bits as part of a subnet?
>
> Mark
| |
| Kevin W. 2002-08-24, 9:28 pm |
| Hello,
The first 16 bits are considered the network mask. Any bits borrowed
from the host address are part of the subnet mask. But the CIDR
notation for the address would be 172.16.10.2/25.
Kevin
Mark Smythe <smythe132@attbi.com> wrote in message news:<3D67930D.8EE61571@attbi.com>...
> Hi All,
> With IP address 172.16.10.2 255.255.255.128 the pretend test I took
> says there are 9 subnet bits in this address. How could this be? It
> looks like 25 bits. Are they insisting this based on Class B? They
> dont count the first 16 bits as part of a subnet?
>
> Mark
| |
| Jonathan 2002-08-24, 11:28 pm |
|
"Mark Smythe" <smythe132@attbi.com> wrote in message
news:3D67930D.8EE61571@attbi.com...
> Hi All,
> With IP address 172.16.10.2 255.255.255.128 the pretend test I took
> says there are 9 subnet bits in this address. How could this be?
Easy.
Because 172.16.10.2 is a class B address, easily noted by the fact that its
most significant bits, in the first octet are 10, a class A address starts
with a 0, a class B starts with a 10, Class C starts 11, D, 111, E, 1111...
Since it is a Class B address, that means the default mask for classful
routing protocols would be 255.255.0.0, or 16 bits of base subnetting.
Class A has 8 bits
Class B has 16 bits
Class C has 24 bits
Since your mask is 255.255.255.128, you get a total of 25 bits of
subnetting, subtracting your standard classful bits, yields you 9 bits of
subnetting from a normal B address.
> It
> looks like 25 bits.
This is the total number of subnet bits, not the classful number of subnet
bits.
> Are they insisting this based on Class B?
Yep.
> They
> dont count the first 16 bits as part of a subnet?
>
Not usually.
This is really beacause of older Cisco IOSs, in the initial configuration
dialog, would ask you for the number of subnet bits for each IP address you
set in.... if you typed 0, you would get the classful mask.
> Mark
>
Jonathan
| |
| Jonathan 2002-08-25, 12:29 am |
|
"Mark Smythe" <smythe132@attbi.com> wrote in message
news:3D67930D.8EE61571@attbi.com...
> Hi All,
> With IP address 172.16.10.2 255.255.255.128 the pretend test I took
> says there are 9 subnet bits in this address. How could this be?
Easy.
Because 172.16.10.2 is a class B address, easily noted by the fact that its
most significant bits, in the first octet are 10, a class A address starts
with a 0, a class B starts with a 10, Class C starts 11, D, 111, E, 1111...
Since it is a Class B address, that means the default mask for classful
routing protocols would be 255.255.0.0, or 16 bits of base subnetting.
Class A has 8 bits
Class B has 16 bits
Class C has 24 bits
Since your mask is 255.255.255.128, you get a total of 25 bits of
subnetting, subtracting your standard classful bits, yields you 9 bits of
subnetting from a normal B address.
> It
> looks like 25 bits.
This is the total number of subnet bits, not the classful number of subnet
bits.
> Are they insisting this based on Class B?
Yep.
> They
> dont count the first 16 bits as part of a subnet?
>
Not usually.
This is really beacause of older Cisco IOSs, in the initial configuration
dialog, would ask you for the number of subnet bits for each IP address you
set in.... if you typed 0, you would get the classful mask.
> Mark
>
Jonathan
| |
| Jonathan 2002-08-25, 12:29 am |
|
"Mark Smythe" <smythe132@attbi.com> wrote in message
news:3D67AB8B.875DA6F2@attbi.com...
>
>
> Seth Corduan wrote:
>
> > I'm by no means an expert, but I feel like taking a crack at this:
> >
> > The IP number is in the 128-191 range, so "technically" that should make
it
> > a class B. (Near as I can tell, the CCNA is all about what something
> > "technically" is.) That would take the first 16 right out. (Again, I
think)
> > and that leaves the 8 for the 3rd 255 and of course the 1 for the 128.
>
> But technically the first 16 bits are part of the subnet.
No, they are the normal classful boundary... a class B address, with a class
B mask is not subnetted at all... if it didn't have those first 16 bits of
mask, then it would be a supernetted address.
e.g.:
172 . 17 . 10
.. 1
^^^^^^^^^^^^^^^^^^^^^^^^ ^ ^^^^^^^^^^^^^^^^^
| |
|
SUPERNET CLASSFUL BOUNDARY SUBNET
> I dont understand how
> being a class B address would eliminate the need to consider those bits
i.e.
> 172.16.10.1 is different subnet than 172.17.10.1 if both are Class B.
A long time ago, in a network far far away, there was only RIP v1... and it
was good, it could do some stuff, but it blew XXX... and here's why:
RIP did not pass mask information along with its routing updates... it used
one constant mask for the entire network, or used classful network
boundaries to determine where network bits stopped, and host bits began.
The problem was that there weren't that many IP addresses around (4 billion
plus just wasn't cutting it...), and if you used classful boundaries, you
wasted some 255 IP addresses for your WAN links...
Eventually, it was realized that we were gonna run out... For a few seconds
everyone panicked, and subnetting was born... It probably ocurred something
like this:
"Hey, there are only 3 IP addresses left!"
"Where did they all go????"
"Well, we wasted 4 million or so on our WAN links..."
"WHAT????"
"Well, we use a class A network, and we can't subnet... so..."
"Subnet then!!!!"
And it was born... Those class B and C addresses got subnetted down to
nothingness, and the universe was saved by just a few 1's...
> But
> the logic of this test would say that there are no subnet bits.
Right. Just regular, good old fashioned Class bits...
> Or maybe they
> try to differentiate between a "network" address or a "subnet" of that
network address. Or maybe I have the
> terminolgy wrong or something.
You do, a little.
> Although I have studied IP and the classes, in
> reality I have never thought of classes when setting up IP schemes. To me
the
> number of network bits starts from the far left . I could also have
> 172.16.10.1 224.0.0.0.
This is called Supernetting and is only used on core routers for aggregating
mass quantities of routes...
> Would a router actually not let me configure an
> address if it is out of class? I appreciate all the help.
>
Depends on the router...
Jonathan
> Mark
>
>
>
> >
> >
> > In short, yes, because the IP is within the Class B range.
> >
> > Corrections? Millimiter, this seems up your alley.
> >
> > Seth C
> >
> > ----- Original Message -----
> > From: "Mark Smythe" <smythe132@attbi.com>
> > Newsgroups: alt.certification.cisco
> > Sent: Saturday, August 24, 2002 9:03 AM
> > Subject: Ok, what is a subnet in the CCNA
> >
> > > Hi All,
> > > With IP address 172.16.10.2 255.255.255.128 the pretend test I took
> > > says there are 9 subnet bits in this address. How could this be? It
> > > looks like 25 bits. Are they insisting this based on Class B? They
> > > dont count the first 16 bits as part of a subnet?
> > >
> > > Mark
> > >
>
| |
| Jonathan 2002-08-25, 12:29 am |
|
"steve harris" <steveharris1@hotmail.com> wrote in message
news:ak8ftu$244p$1@feed.centurytel.net...
> Mark Smythe wrote:
>
> >
> >
> > Seth Corduan wrote:
> >
> >> I'm by no means an expert, but I feel like taking a crack at this:
> >>
> >> The IP number is in the 128-191 range, so "technically" that should
make[
color=darkred]
> >> it a class B. (Near as I can tell, the CCNA is all about what something
> >> "technically" is.) That would take the first 16 right out. (Again, I
> >> think) and that leaves the 8 for the 3rd 255 and of course the 1 for[/color]
the[c
olor=darkred]
> >> 128.
> >
> > But technically the first 16 bits are part of the subnet. I dont
> > understand how being a class B address would eliminate the need to
> > consider those bits i.e.
> > 172.16.10.1 is different subnet than 172.17.10.1 if both are Class B[/color]
..
> > But the logic of this test would say that there are no subnet bits. Or
> > maybe they try to differentiate between
> > a "network" address or a "subnet" of that network address. Or maybe I
have
> > the terminolgy wrong or something. Although I have studied IP and the
> > classes, in reality I have never thought of classes when setting up IP
> > schemes. To me the number of network bits starts from the far left . I
> > could also have
> > 172.16.10.1 224.0.0.0. Would a router actually not let me configure
an
> > address if it is out of class? I appreciate all the help.
> >
> > Mark
> >
> >
> >
> >>
> >>
> >> In short, yes, because the IP is within the Class B range.
> >>
> >> Corrections? Millimiter, this seems up your alley.
> >>
> >> Seth C
> >>
> >> ----- Original Message -----
> >> From: "Mark Smythe" <smythe132@attbi.com>
> >> Newsgroups: alt.certification.cisco
> >> Sent: Saturday, August 24, 2002 9:03 AM
> >> Subject: Ok, what is a subnet in the CCNA
> >>
> >> > Hi All,
> >> > With IP address 172.16.10.2 255.255.255.128 the pretend test I took
> >> > says there are 9 subnet bits in this address. How could this be? It
> >> > looks like 25 bits. Are they insisting this based on Class B? They
> >> > dont count the first 16 bits as part of a subnet?
> >> >
> >> > Mark
> >> >
>
> Mark,
> the short answer is it's the way Cisco does things.
>
> Like if you are using a class B address, it assumes 16 bits already on the
> mask. You enter only enter the additional number of bits. It's the way the
> Cisco routers are set up.
>
> Wait until you set up OSPF on a Cisco router, you have to put in the mask
> backwards. "0.0.255.255"
>
0.0.255.255 is NOT a backwards subnet mask... If you think about it like
that, you will be totally screwed up when it comes to doing wildcard
masks...
The wildcard bits are care/don't care bits...
Let's say you have a network of 10.5.8.0/24 and you want OSPF (or anything
else for that matter, when you get to ACLs you will know what I mean) to
advertise that network, you would give it a:
network 10.5.8.0 0.0.0.255 area 0
What if your mask is not 24, but 29? And you have 10.5.8.16/29?
Then your network statement looks like this:
network 10.5.8.16 0.0.0.15 area 0
It is actually much more complicated then this, but we'll get to that
later...
> Steve
Jonathan
| |
| Jonathan 2002-08-25, 1:29 am |
|
"Mark Smythe" <smythe132@attbi.com> wrote in message
news:3D67AB8B.875DA6F2@attbi.com...
>
>
> Seth Corduan wrote:
>
> > I'm by no means an expert, but I feel like taking a crack at this:
> >
> > The IP number is in the 128-191 range, so "technically" that should make
it
> > a class B. (Near as I can tell, the CCNA is all about what something
> > "technically" is.) That would take the first 16 right out. (Again, I
think)
> > and that leaves the 8 for the 3rd 255 and of course the 1 for the 128.
>
> But technically the first 16 bits are part of the subnet.
No, they are the normal classful boundary... a class B address, with a class
B mask is not subnetted at all... if it didn't have those first 16 bits of
mask, then it would be a supernetted address.
e.g.:
172 . 17 . 10
.. 1
^^^^^^^^^^^^^^^^^^^^^^^^ ^ ^^^^^^^^^^^^^^^^^
| |
|
SUPERNET CLASSFUL BOUNDARY SUBNET
> I dont understand how
> being a class B address would eliminate the need to consider those bits
i.e.
> 172.16.10.1 is different subnet than 172.17.10.1 if both are Class B.
A long time ago, in a network far far away, there was only RIP v1... and it
was good, it could do some stuff, but it blew XXX... and here's why:
RIP did not pass mask information along with its routing updates... it used
one constant mask for the entire network, or used classful network
boundaries to determine where network bits stopped, and host bits began.
The problem was that there weren't that many IP addresses around (4 billion
plus just wasn't cutting it...), and if you used classful boundaries, you
wasted some 255 IP addresses for your WAN links...
Eventually, it was realized that we were gonna run out... For a few seconds
everyone panicked, and subnetting was born... It probably ocurred something
like this:
"Hey, there are only 3 IP addresses left!"
"Where did they all go????"
"Well, we wasted 4 million or so on our WAN links..."
"WHAT????"
"Well, we use a class A network, and we can't subnet... so..."
"Subnet then!!!!"
And it was born... Those class B and C addresses got subnetted down to
nothingness, and the universe was saved by just a few 1's...
> But
> the logic of this test would say that there are no subnet bits.
Right. Just regular, good old fashioned Class bits...
> Or maybe they
> try to differentiate between a "network" address or a "subnet" of that
network address. Or maybe I have the
> terminolgy wrong or something.
You do, a little.
> Although I have studied IP and the classes, in
> reality I have never thought of classes when setting up IP schemes. To me
the
> number of network bits starts from the far left . I could also have
> 172.16.10.1 224.0.0.0.
This is called Supernetting and is only used on core routers for aggregating
mass quantities of routes...
> Would a router actually not let me configure an
> address if it is out of class? I appreciate all the help.
>
Depends on the router...
Jonathan
> Mark
>
>
>
> >
> >
> > In short, yes, because the IP is within the Class B range.
> >
> > Corrections? Millimiter, this seems up your alley.
> >
> > Seth C
> >
> > ----- Original Message -----
> > From: "Mark Smythe" <smythe132@attbi.com>
> > Newsgroups: alt.certification.cisco
> > Sent: Saturday, August 24, 2002 9:03 AM
> > Subject: Ok, what is a subnet in the CCNA
> >
> > > Hi All,
> > > With IP address 172.16.10.2 255.255.255.128 the pretend test I took
> > > says there are 9 subnet bits in this address. How could this be? It
> > > looks like 25 bits. Are they insisting this based on Class B? They
> > > dont count the first 16 bits as part of a subnet?
> > >
> > > Mark
> > >
>
| |
| Jonathan 2002-08-25, 1:29 am |
|
"steve harris" <steveharris1@hotmail.com> wrote in message
news:ak8ftu$244p$1@feed.centurytel.net...
> Mark Smythe wrote:
>
> >
> >
> > Seth Corduan wrote:
> >
> >> I'm by no means an expert, but I feel like taking a crack at this:
> >>
> >> The IP number is in the 128-191 range, so "technically" that should
make[
color=darkred]
> >> it a class B. (Near as I can tell, the CCNA is all about what something
> >> "technically" is.) That would take the first 16 right out. (Again, I
> >> think) and that leaves the 8 for the 3rd 255 and of course the 1 for[/color]
the[c
olor=darkred]
> >> 128.
> >
> > But technically the first 16 bits are part of the subnet. I dont
> > understand how being a class B address would eliminate the need to
> > consider those bits i.e.
> > 172.16.10.1 is different subnet than 172.17.10.1 if both are Class B[/color]
..
> > But the logic of this test would say that there are no subnet bits. Or
> > maybe they try to differentiate between
> > a "network" address or a "subnet" of that network address. Or maybe I
have
> > the terminolgy wrong or something. Although I have studied IP and the
> > classes, in reality I have never thought of classes when setting up IP
> > schemes. To me the number of network bits starts from the far left . I
> > could also have
> > 172.16.10.1 224.0.0.0. Would a router actually not let me configure
an
> > address if it is out of class? I appreciate all the help.
> >
> > Mark
> >
> >
> >
> >>
> >>
> >> In short, yes, because the IP is within the Class B range.
> >>
> >> Corrections? Millimiter, this seems up your alley.
> >>
> >> Seth C
> >>
> >> ----- Original Message -----
> >> From: "Mark Smythe" <smythe132@attbi.com>
> >> Newsgroups: alt.certification.cisco
> >> Sent: Saturday, August 24, 2002 9:03 AM
> >> Subject: Ok, what is a subnet in the CCNA
> >>
> >> > Hi All,
> >> > With IP address 172.16.10.2 255.255.255.128 the pretend test I took
> >> > says there are 9 subnet bits in this address. How could this be? It
> >> > looks like 25 bits. Are they insisting this based on Class B? They
> >> > dont count the first 16 bits as part of a subnet?
> >> >
> >> > Mark
> >> >
>
> Mark,
> the short answer is it's the way Cisco does things.
>
> Like if you are using a class B address, it assumes 16 bits already on the
> mask. You enter only enter the additional number of bits. It's the way the
> Cisco routers are set up.
>
> Wait until you set up OSPF on a Cisco router, you have to put in the mask
> backwards. "0.0.255.255"
>
0.0.255.255 is NOT a backwards subnet mask... If you think about it like
that, you will be totally screwed up when it comes to doing wildcard
masks...
The wildcard bits are care/don't care bits...
Let's say you have a network of 10.5.8.0/24 and you want OSPF (or anything
else for that matter, when you get to ACLs you will know what I mean) to
advertise that network, you would give it a:
network 10.5.8.0 0.0.0.255 area 0
What if your mask is not 24, but 29? And you have 10.5.8.16/29?
Then your network statement looks like this:
network 10.5.8.16 0.0.0.15 area 0
It is actually much more complicated then this, but we'll get to that
later...
> Steve
Jonathan
| |
| beemer 2002-08-25, 5:28 pm |
| Yes it is Class B and the first 16 bits are not counted as subnets. Cisco
refers to subnets as the borrowed portion. 172.16.10.2 being a class B (see
chart below)would have a natural classful subnet mask of 255.255.0.0. Based
on the subnet mask you would count the bits added to the mask above the
default classful mask to get the borrowed bits. In this cast it 255.128 or
11111111.10000000. Count the ones you get 9 ones and that means 9 bits of
subnetting. Clear as mud? Probably but you should be able to do this in
your head after a while.
Class A 1.0.0.0 through 126.0.0.0 255.0.0.0 16,777,214
Class B 128.1.0.0 through 191.254.0.0 255.255.0.0 65,534
Class C 192.0.1.0 through 223.255.254.0 255.255.255.0 254
Class D 224.0.0.0 through 239.255.255.254 Used Multicast No host range
exists
Class E Research Research No host range exists
Other 127.0.0.0 255.0.0.0 Reserved to LOCALHOST and Loop back testing
Mark Smythe <smythe132@attbi.com> wrote in message
news:3D67930D.8EE61571@attbi.com...
> Hi All,
> With IP address 172.16.10.2 255.255.255.128 the pretend test I took
> says there are 9 subnet bits in this address. How could this be? It
> looks like 25 bits. Are they insisting this based on Class B? They
> dont count the first 16 bits as part of a subnet?
>
> Mark
>
| |
| beemer 2002-08-25, 6:28 pm |
| Yes it is Class B and the first 16 bits are not counted as subnets. Cisco
refers to subnets as the borrowed portion. 172.16.10.2 being a class B (see
chart below)would have a natural classful subnet mask of 255.255.0.0. Based
on the subnet mask you would count the bits added to the mask above the
default classful mask to get the borrowed bits. In this cast it 255.128 or
11111111.10000000. Count the ones you get 9 ones and that means 9 bits of
subnetting. Clear as mud? Probably but you should be able to do this in
your head after a while.
Class A 1.0.0.0 through 126.0.0.0 255.0.0.0 16,777,214
Class B 128.1.0.0 through 191.254.0.0 255.255.0.0 65,534
Class C 192.0.1.0 through 223.255.254.0 255.255.255.0 254
Class D 224.0.0.0 through 239.255.255.254 Used Multicast No host range
exists
Class E Research Research No host range exists
Other 127.0.0.0 255.0.0.0 Reserved to LOCALHOST and Loop back testing
Mark Smythe <smythe132@attbi.com> wrote in message
news:3D67930D.8EE61571@attbi.com...
> Hi All,
> With IP address 172.16.10.2 255.255.255.128 the pretend test I took
> says there are 9 subnet bits in this address. How could this be? It
> looks like 25 bits. Are they insisting this based on Class B? They
> dont count the first 16 bits as part of a subnet?
>
> Mark
>
| |
|
| Hi Mark,
No worries, it is confusing, but I think they mean an extra 9 bits from the
"origional" clasfull network boundary, for example,
network ID 172.16.0.0 = 16 bits (255.255.0.0) + 9 extra bits = 25 bits,
Therefore, this gives, 255.255.255.128
If you any of you guys are stuck for CCNA / CCNP or CCIE written mentoring,
drop me a line at yahoo messenger, id lanbase_01
I do a lot of e-training for cisco at reasonable rates :-)
Hope that clears it up for you,
Regards,
Mal
"Mark Smythe" <smythe132@attbi.com> wrote in message
news:3D67930D.8EE61571@attbi.com...
> Hi All,
> With IP address 172.16.10.2 255.255.255.128 the pretend test I took
> says there are 9 subnet bits in this address. How could this be? It
> looks like 25 bits. Are they insisting this based on Class B? They
> dont count the first 16 bits as part of a subnet?
>
> Mark
>
| |
|
| Hi Mark,
No worries, it is confusing, but I think they mean an extra 9 bits from the
"origional" clasfull network boundary, for example,
network ID 172.16.0.0 = 16 bits (255.255.0.0) + 9 extra bits = 25 bits,
Therefore, this gives, 255.255.255.128
If you any of you guys are stuck for CCNA / CCNP or CCIE written mentoring,
drop me a line at yahoo messenger, id lanbase_01
I do a lot of e-training for cisco at reasonable rates :-)
Hope that clears it up for you,
Regards,
Mal
"Mark Smythe" <smythe132@attbi.com> wrote in message
news:3D67930D.8EE61571@attbi.com...
> Hi All,
> With IP address 172.16.10.2 255.255.255.128 the pretend test I took
> says there are 9 subnet bits in this address. How could this be? It
> looks like 25 bits. Are they insisting this based on Class B? They
> dont count the first 16 bits as part of a subnet?
>
> Mark
>
| |
| George Hand 2002-08-26, 9:29 pm |
| Here's my tuppence worth...
The thing to remember is that masks in classful addressing are NETWORK MASKS
(sorry, I'm not shouting-just trying to make it stand out), and when we
needed more networks, we 'borrowed' bits from the host portion....creating
subnets! Then the mask is a SUBNET MASK....Cisco regards setting up
addressing as being based on the network mask...i.e. the question in the
setup dialogue about "how many subnet bits are you using"....if you reply 0,
you get the standard network mask such as 172.16.0.0 255.255.0.0.....if you
want to use subnets e.g. 172.16.0.0 255.255.192.0, then the subnet bits you
use in reply to the question is 2....in other words, you are using 18 bits
of network and 14 bits of host addressing.
I hope that makes it clearer...
Regards,
George
"mal" <mal_1024@hotmail.com> wrote in message
news:akd13u$d8m$1@newsg2.svr.pol.co.uk...
> Hi Mark,
>
> No worries, it is confusing, but I think they mean an extra 9 bits from
the
> "origional" clasfull network boundary, for example,
>
> network ID 172.16.0.0 = 16 bits (255.255.0.0) + 9 extra bits = 25 bits,
>
> Therefore, this gives, 255.255.255.128
>
> If you any of you guys are stuck for CCNA / CCNP or CCIE written
mentoring,
> drop me a line at yahoo messenger, id lanbase_01
>
> I do a lot of e-training for cisco at reasonable rates :-)
>
> Hope that clears it up for you,
> Regards,
> Mal
>
> "Mark Smythe" <smythe132@attbi.com> wrote in message
> news:3D67930D.8EE61571@attbi.com...
> > Hi All,
> > With IP address 172.16.10.2 255.255.255.128 the pretend test I took
> > says there are 9 subnet bits in this address. How could this be? It
> > looks like 25 bits. Are they insisting this based on Class B? They
> > dont count the first 16 bits as part of a subnet?
> >
> > Mark
> >
>
>
| |
| George Hand 2002-08-26, 10:28 pm |
| Here's my tuppence worth...
The thing to remember is that masks in classful addressing are NETWORK MASKS
(sorry, I'm not shouting-just trying to make it stand out), and when we
needed more networks, we 'borrowed' bits from the host portion....creating
subnets! Then the mask is a SUBNET MASK....Cisco regards setting up
addressing as being based on the network mask...i.e. the question in the
setup dialogue about "how many subnet bits are you using"....if you reply 0,
you get the standard network mask such as 172.16.0.0 255.255.0.0.....if you
want to use subnets e.g. 172.16.0.0 255.255.192.0, then the subnet bits you
use in reply to the question is 2....in other words, you are using 18 bits
of network and 14 bits of host addressing.
I hope that makes it clearer...
Regards,
George
"mal" <mal_1024@hotmail.com> wrote in message
news:akd13u$d8m$1@newsg2.svr.pol.co.uk...
> Hi Mark,
>
> No worries, it is confusing, but I think they mean an extra 9 bits from
the
> "origional" clasfull network boundary, for example,
>
> network ID 172.16.0.0 = 16 bits (255.255.0.0) + 9 extra bits = 25 bits,
>
> Therefore, this gives, 255.255.255.128
>
> If you any of you guys are stuck for CCNA / CCNP or CCIE written
mentoring,
> drop me a line at yahoo messenger, id lanbase_01
>
> I do a lot of e-training for cisco at reasonable rates :-)
>
> Hope that clears it up for you,
> Regards,
> Mal
>
> "Mark Smythe" <smythe132@attbi.com> wrote in message
> news:3D67930D.8EE61571@attbi.com...
> > Hi All,
> > With IP address 172.16.10.2 255.255.255.128 the pretend test I took
> > says there are 9 subnet bits in this address. How could this be? It
> > looks like 25 bits. Are they insisting this based on Class B? They
> > dont count the first 16 bits as part of a subnet?
> >
> > Mark
> >
>
>
| |
| george 2002-08-30, 8:28 pm |
| > Mark,
....
> Wait until you set up OSPF on a Cisco router, you have to put in the mask
> backwards. "0.0.255.255"
>
> Steve
A useful tip (I hope) re wildcard bits...
A wildcard mask and the equivalent standard mask always add up to
255.255.255.255.
For example, if I have a 255.255.255.128 mask, then the equivalent
wildcard bits would be 0.0.0.127.
A mask of 255.255.255.240 equates to wildcard bits 0.0.0.15
A mask of 240.0.0.0 equates to wildcard bits 15.255.255.255
Again, the A.B.C.Ds always add up to 255.255.255.255
| |
| george 2002-08-30, 9:28 pm |
| > Mark,
....
> Wait until you set up OSPF on a Cisco router, you have to put in the mask
> backwards. "0.0.255.255"
>
> Steve
A useful tip (I hope) re wildcard bits...
A wildcard mask and the equivalent standard mask always add up to
255.255.255.255.
For example, if I have a 255.255.255.128 mask, then the equivalent
wildcard bits would be 0.0.0.127.
A mask of 255.255.255.240 equates to wildcard bits 0.0.0.15
A mask of 240.0.0.0 equates to wildcard bits 15.255.255.255
Again, the A.B.C.Ds always add up to 255.255.255.255
|
|
|
|
|