split/isolate network

[rikona]

I'd like to split my local net into two parts which are completely
isolated, with no possibility of direct communication between them.
I'm wondering how to do this with a linux box, perhaps as follows:

 cable modem -> router -> linux box -> 2 isolated net connections

 I'm not sure what this might be called, and google was not my friend
 re this problem, so I thought I'd ask here. The linux box would be
 dedicated, not used for other purposes, and would be an older, much-
 less-capable-hdwe box.

 One of the net connections [side 1] would have several fixed-IP boxes
 on it, with NO other box addresses allowed. The other [side 2] would
 need DHCP, with one or more boxes connected, whose address range does
 NOT overlap that of side 1. [I'm thinking 192.168... and 10.0... for
 example.]

 I'm not sure how to do this, but am assuming it is likely possible.
 Any suggestions for how to do this, or where to find out on the net,
 would be appreciated.

 Also, it looks like 10.04 will not install on the less-capable-hdwe
 box. If you know of alternative S/W that might work on older hdwe,
 please let me know. And, if there's a very inexpensive hardware
 solution that would do the above, that might be preferred, since I'm
 far from being a linux/network guru. :-)

 Many thanks,

  rikona