Next Meeting: July 7: Social gathering Next Installfest: TBD Latest News: Jun. 14: June LUGOD meeting cancelled Page last updated: 2001 Dec 30 17:01

• From: Dale Bewley <dale@bMAPSewley.net>
• Date: Tue, 20 Feb 2001 14:08:46 -0800
• References: 20010220134714.A32719@dirac.org

```I have a hard time explaining it to other people, but
you have to picture the binary number in your head and after a while you
memorize most of these.

To convert /30 to the mask I say to myself:
32-30=2 bits
2 bits can create a maximum of 4 in decimal
256-4=252

Once you do enough of that, you quickly see 252 and think /30.
Or see 240 and think /28. (256-240=16 which is 2^4 and 32-4=28)

255.255.254.0 would be /23 meaning 23 bits in the netmask.
If you subtract 254 from 256 you'll get 2. That tells you you have two
/24 subnetworks.

It gets more complicated when you are peeling off a /29 or something in
the middle of the range from 0-255. You have to make sure you network
starts on a boundary divisible by 8. (32-29=3bits = max of 8)

is the subnet?
It's 1.2.3.79 and 1.2.3.72. The next subnet starts at .80 so the

Sorry if that wasn't much help.
If anyone has a easier way to explain that, please do.
I probably should have just kept my mouth shut. :)

On Tue, 20 Feb 2001, Peter Jay Salzman wrote:
> is there a way to see, without converting numbers to binary and performing a
> a bitwise and, that:
>
>       131.155.72.0/255.255.254.0
>
> means all addresses from 131.155.72.0 through 131.155.73.255?  is there
> short cut for this?
>
> something like "255 means that quad is firm, 254 means you can go one
> higher, and 0 means the quad can be anything"?  i know that's "loose talk",
> but is it generally true?
>
> pete
>
> --
> "It's better to be safe than assimilated."                   p@dirac.org
>                       -- Chakotay                            www.dirac.org/p
>

--
Dale Bewley - Bewley Internet Solutions Inc. http://bewley.net/
```