Re: communicating between subnets

[David Woyciesjes <dwoyciesjes@comcast.net>]

David Z Maze wrote:
> 
> David Woyciesjes <dwoyciesjes@comcast.net> writes:
> 
> > Praveen Kallakuri wrote:
> >
> >> secondly, how can i split 1.x into different subnets?
> >
> >       I'll have to look it up, but it involves using a subnet mask like
> > 255.255.224.0 instead of the usual 255.255.255.0. This way you can have
> > a subnet with space for only a few machines, instead of the 254 spots
> > you get with using 192.168.1.x with 255.255.255.0 as the subnet mask.
> 
> If you wanted four identical networks -- say, 192.168.1.0/26,
> 192.168.1.64/26, 192.168.1.128/26, and 192.168.1.192/26 -- you'd set
> the netmask to 255.255.255.192.

	But, do you lose the first and last network ranges?

	And, the usual 192.168.1.x IP range (with 255.255.255.0 subnet mask)
that most people use for home, can be written shorthand as
192.168.1.x/24.

> Q: Is 192.168.1.60 on 192.168.1.0/26?
> 
> A: See if a bitwise AND of the address in question and the netmask
> equals the network address:
> 
>       192.168.  1. 60
>   AND 255.255.255.192
>       ---------------
>       192.168.  1.  0 <== YES!
> 
> Q: What does 192.168.1.128/26 mean?
> 
> A: It means that the network address is 192.168.1.128, and the netmask
> is a 32-bit word with the high 26 bits set.  These get broken up into
> four groups of eight ("octets"); the three highest octets have all
> eight bits set, for decimal 255, and the last is 11000000, for decimal
> 128+64=192.

	But, for the 192.168.1.x/26 network subnet, 192.168.1.128 can't be used
for a computer in that subnet, because it's the address for the subnet
section.
	Rephrased...
Each IP Address subnet range, (say, 192.168.1.x with a subnet mask of
255.255.255.0) has a network address, which is the first IP address of
the subnet- 192.168.1.0 in this case. And the last IP address is the
broadcast address for that subnet - 192.168.1.255. That leaves 254
address for machines on that subnet.

	Since we're using a 192.168.1.x/26 subnet, it gets broken down into 4
subnets:
network address - broadcast address
192.168.1.0     - 192.168.1.63
192.168.1.64    - 192.168.1.127
192.168.1.128   - 192.168.1.191
192.168.1.192   - 192.168.1.255
...which leaves only 62 available IP addresses in each subnet.

	Here's how this applies to home use...
	Say your home network has 30 machines. Assign them address between
192.168.1.129 and 192.168.1.190, and use 192.168.1.192 for the subnet
mask. Now you can safely assume that any traffic from a 192.168.1.x
address, that is outside your subnet, is spoofed. Or something like
that... :)
	Also, you can create even smaller subnets if you like, too. I think the
smallest usable subnet only allows 6 IP addresses.
	Yep, I'll definitely bring my books with me tomorrow...

--
---   Dave Woyciesjes
---   ICQ# 905818