Re: [SLUG] subnetting a subnet

[DaZZa <dazza@xxxxxxxxxx>]

On Mon, 17 Jul 2000, Terry Collins wrote:

> Well, it is Monday morning and what the heck. I'm not sure how it
> goes, but this is how I think it goes, which someone can correct if
> wrong.

Right idea, wrong subnet masks. :-)

> 1 block    
>        192.168.1.0-255       netmask  255.255.255.0

That one's OK.

> 2 blocks
>        192.168.1.0-127       netmask  255.255.255.128
>        192.168.1.128-255     netmask  255.255.255.128

That one too.

> 4 blocks 
>        192.168.1.0-63        netmask  255.255.255.64
>        192.168.1.64-127      netmask  255.255.255.64
>        192.168.1.128-191     netmask  255.255.255.64
>        192.168.1.192-255     netmask  255.255.255.64

This one should be 255.255.255.192

> 8 blocks
>        192.168.1.0-31        netmask  255.255.255.32
>        192.168.1.32-63       netmask  255.255.255.32
>        192.168.1.64-95       netmask  255.255.255.32
>        192.168.1.96-127      netmask  255.255.255.32 
>        192.168.1.128-159     netmask  255.255.255.32
>        192.168.1.160-191     netmask  255.255.255.32 
>        192.168.1.192-223     netmask  255.255.255.32
>        192.168.1.224-255     netmask  255.255.255.32

This one should be 255.255.255.224.

The next one is a subnet mask of 255.255.255.240, then 255.255.255.248,
and the last option is 255.255.255.252. All for a class C address, of
course - there are many more options with class A and B networks. Subnet
masking consists of _adding_ bits to the mask, not taking them away.

Things to note when subnetting.

You loose 2 addresses - the first and the last - from every subnet. The
first becomes the "network" address, and the last becomes the "broadcast"
address. So, with a netmask of 255.255.255.252 which, if you work it out,
gives 4 valid IP hosts, you can only use _two_ of them - this is very
effective netmasking for point-to-point links {not dialups - dedicated
services, for example}, as it allows you to use a whole class C to connect
as many as 120 odd sites to your "core" with the one address range.

You can do some nifty stuff by mixing subnet masks in one address range.
You could use a 64 node block {255.255.255.192 subnet mask} in your
office, then use the next "64 address" block but subnet it down to
255.255.255.252 to connect remote sites, then use the third "64 address"
block at another site which connects to YOU, and through you to the net,
or someone else.

Subnet masking is an art, not a science. :)

DaZZa