I have a switch that is connected to several clients and a router and i want to make one of the interfaces connect directly to a nic on another machine without being affected by the other computers on the switch like other computers not being able to ping it. also how does it distinguish between nics being used to connect to the switch (the nic being the one that is on one of the computers that you want to connect to, not on the switch itself)

i am using FreeBSD 8.2 but i would also like to know how to do this in Debian and OpenBSD too

Well first of all, this topic would be more appropriate in the Networking forum since it isn't a BSD issue; a moderator can move it for you.

As for the question, if I am understanding you correctly, the closest thing to what you are looking for would be a VLAN. This lets you setup virtual networks within the switch to segment different hosts on the network. Unfortunately, to the best of my knowledge, VLANs need to be setup and supported by the switch itself. So unless you have an enterprise level switch, you won't be able to enable it.

A true switch (and most are) is a device that connects directly as opposed to a hub. You do nothing and it works. Other than limiting the arp and other chatter and ping it is as direct as it gets with a device. You could just use a new subnet that is out of the others.

However if you do have a so called smart switch then you may have options like VLAN or miror or such.