One Nic; Multiple Subnets

[Mike <1100100@gmail.com>]

I've been looking through the monthly archives of this list, but I
can't find the needle in the haystack.  I saw this question answered
before, and I'm hoping I'll see it again.  :-)

I have an old slackware routerbox that only has room for 2 nics.
Right now there are two nics. in it and they are set up like so:

eth0 --> Internet (Dynamic IP: Assigned by ISP)
eth1 --> Lan  (Gateway Interface:  192.168.1.1)

I will soon be joining some computers from another LAN into the one
mentioned above.
I will need to set up security measures so that the new computers will
not be hacked or viewed by the other users on the LAN.

Even though I've only got one C-Class subnet (192.168.1.1 - 255), I
want to create 2 or more "virtual" subnets to reside in this address
range.

How do I create the multiple subnets?
Do I need to use route command or ipsec.?
And what would the iptables rule look like, where Subnet "B" rejects
all packets coming from Subnet "A"?

Is this even close? ---
$IPTABLES -t filter FORWARD -A -i eth1 -s 192.168.1.2/150
--to-destination 192.168.1.151/253 -j DENY

Thank you for your time and help.

Mike