21 bit subnet ?

I'm confused. A colleage has requested I number a network 10.6.0.0/21

that's a 21 bit subnet, 255.255.248.0, right?

'cause I'm (obviously) no expert and I'm looking around the net I see that
this might just be another way to describe a 24bit netmask (255.255.255.0)

What's the crack? Can anyone explain it to me?

thanks,


Carl

Sgranocchiando il cranio di Carl Farrington, vi trovai inciso:

> I'm confused. A colleage has requested I number a network 10.6.0.0/21
>
> that's a 21 bit subnet, 255.255.248.0, right?
>
> 'cause I'm (obviously) no expert and I'm looking around the net I see
> that this might just be another way to describe a 24bit netmask
> (255.255.255.0)

no, it isn't...

> What's the crack? Can anyone explain it to me?

try converting the netmask in binary and i think everything becomes
clearer...

c ya

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"Carl Farrington" <(E-Mail Removed)> wrote in message
news:bgt66c$4po$1$(E-Mail Removed)

> I'm confused. A colleage has requested I number a network 10.6.0.0/21
> that's a 21 bit subnet, 255.255.248.0, right?
> 'cause I'm (obviously) no expert and I'm looking around the net I see
> that this might just be another way to describe a 24bit netmask
> (255.255.255.0)
> What's the crack? Can anyone explain it to me?

Look at the number of hosts addressable in /21 and /24:

$ ./ipcalc.pl 10.6.0.0/21
Address: 10.6.0.0 00001010.00000110.00000 000.00000000
Netmask: 255.255.248.0 = 21 11111111.11111111.11111 000.00000000
Wildcard: 0.0.7.255 00000000.00000000.00000 111.11111111
Network: 10.6.0.0/21 00001010.00000110.00000 000.00000000
=> (Class A)
Broadcast: 10.6.7.255 00001010.00000110.00000 111.11111111
HostMin: 10.6.0.1 00001010.00000110.00000 000.00000001
HostMax: 10.6.7.254 00001010.00000110.00000 111.11111110
Hosts/Net: 2046 (Private Internet RFC 1918)

$ ./ipcalc.pl 10.6.0.0/24
Address: 10.6.0.0 00001010.00000110.00000000 .00000000
Netmask: 255.255.255.0 = 24 11111111.11111111.11111111 .00000000
Wildcard: 0.0.0.255 00000000.00000000.00000000 .11111111
Network: 10.6.0.0/24 00001010.00000110.00000000 .00000000
=> (Class A)
Broadcast: 10.6.0.255 00001010.00000110.00000000 .11111111
HostMin: 10.6.0.1 00001010.00000110.00000000 .00000001
HostMax: 10.6.0.254 00001010.00000110.00000000 .11111110
Hosts/Net: 254 (Private Internet RFC 1918)



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Dans sa prose, Carl Farrington nous ecrivait :
> I'm confused. A colleage has requested I number a network 10.6.0.0/21
> that's a 21 bit subnet, 255.255.248.0, right?

True.

> 'cause I'm (obviously) no expert and I'm looking around the net I see
> that this might just be another way to describe a 24bit netmask
> (255.255.255.0)

Nope.

> What's the crack? Can anyone explain it to me?

An IP address is composed of a network part and a host part. Netmask is
used to deduce network address (i.e. IP for which host part is 0) from IP.
In short, just operate a AND between IP and mask (binary representation).
The result you get is network address.

It is quite simple with classical 8, 16 and 24 bits masks, as they just
cut network part on a dot in classical representation. Other masks are a
bit more tricky to handle as you can see.

A 24 bits mask lets 8 bits for hosts. So you can address 254 hosts. A 21
bits mask allows you to address 2046 hosts, as its network part is smaller.

Debian has a tool called netmask that can help you in calculating ranges
and netmasks, both classical, CIDR or binary representation. Good tool for
learning

For your present case :

cbr@elendil:~$ netmask -r 10.6.0.0/21
10.6.0.0-10.6.7.255 (2048)

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Carl Farrington wrote:

> I'm confused. A colleage has requested I number a network 10.6.0.0/21
>
> that's a 21 bit subnet, 255.255.248.0, right?
>
> 'cause I'm (obviously) no expert and I'm looking around the net I see that
> this might just be another way to describe a 24bit netmask (255.255.255.0)
>
> What's the crack? Can anyone explain it to me?

That's a legitemate net mask. The local network will then allow 2046
addresses. A netmask does not have to fall on 8 bit boundaries.
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