Networking question

Could I set my network up like this?:
  Modem -- Switch -- Computers
My current setup is like this:
  Modem -- [Computer, Router -- Computers]

What I really wonder is if DHCP will work with several computers on the same port on the modem (which has four ports).  If I had a switch I could try this of course, but the case is that I do not have one (and I do not think I could configure my router into being a switch).

Modem -- Router -- Computers

what's wrong with that?

>>2
I want global IPs (v4 and v6) for all my computers.

Oh, and I'm not going to connect all my computers to the modem as I would like to have one of them as a firewall right after the modem.

[sigh]
if your modem has four ports (i'll bet it has five), it's a modem/router/switch all in one.

 the four ports are ones you're supposed to hook your computers into.  the fifth port, labeled Internet or WAN, is what your dsl goes into (you probably don't have cable).

 since it's a router, it has dhcp built into it.

 dhcp will work with all your computers.  the job of dhcp is to hand out free ip addresses to computers who ask for it (they do if you tell it to "Automatically get IP address" in the Network Connection control panel).  dhcp will hand out as many addresses as you tell it that it may.  you probably can't configure this aspect of dhcp on your hardware router though, but still, it should take care of it.

 check your device configuration for something called "bridged mode", that would effectively turn off the router part.  i don't think you can do that since yours is also a built in switch, and you need a router before the switch for anything to work.

 let me tell you this too: as long as your ISP gives you only one IP, there is no way to give computers behind it their own global, external IPs.

 but i recommend that you need to do the following:

 - learn linux
 - get an old computer (500Mhz+)
 - install two network cards
 - learn how to set it up as a router/firewall

My modem is actually a computer with linux/busybox installed (with ARM processor).
You can login via telnet as root and screw everything up. Only root filesystem is in memory so it would work fine after you reset it.

>>5
since it's a router, it has dhcp built into it.
I see, I thought that was somewhere inside the ISP's network.
check your device configuration for something called "bridged mode", that would effectively turn off the router part.  i don't think you can do that since yours is also a built in switch, and you need a router before the switch for anything to work.
Wouldn't my modem work as a router (this is my original question)?
let me tell you this too: as long as your ISP gives you only one IP, there is no way to give computers behind it their own global, external IPs.
Well, they give me at least four IPs.
- learn linux - get an old computer (500Mhz+) - install two network cards - learn how to set it up as a router/firewall
I've got one running OpenBSD (333MHz and 64MB RAM, need to get a second NIC though, or perhaps two gigabit NICs instead, 100MB is excruciatingly slow in a LAN... hm, one gigabit NIC is enough) -- could I use that one just as a firewall and let my modem handle the routing?

Ah, I remembered that there is an unofficial FAQ for my ISP -- this issue is covered there (I think), and it is claimed that simply using a switch should work.

man pf

>>9
Is that the only packet filter OpenBSD has?  FreeBSD has a whole bunch of different systems.

FreeBSD kicks ass.

just thought i'd drop that in. moving on...

>>10
pf is the only one that matters.

>>7
Well, your router gets its IP from your ISP's DHCP (unless you have a static IP).  The normal configuration is for your router to boot up and then check the network for an IP. 

In a similar fashion, your computer gets its IP from the router's DHCP.

If your modem has the ports I described (1 WAN + 4 LAN), then yes, it's a router and is probably routing as we speak.

however, if your ISP gives you four static IPs, you don't need a router at all.  your basic setup is: internet -> (firewall would go here) -> switch -> computers.

you will need to disable the routing function in your modem/router/switch, if you can.  you will then need to manually assign each ip to each one of your computers, and then connect each to the switch.  using this setup each computer needs its own firewall.

notice in the above diagram that the firewall needs to go between the internet and the switch.  if you can disable the routing function in your modem, you can use your bsd machine to act as a firewall.  you will then need another switch after the firewall, i.e.

 internet wire -> modem/router with routing disabled -> firewall computer -> switch -> other computers

what model of modem do you have?

>>13
[snip]
My IPs are dynamic.
internet wire -> modem/router with routing disabled -> firewall computer -> switch -> other computers
In case my IPs are dynamic, isn't this exactly the same (except routing is enabled)?
what model of modem do you have?
It is a XAVi x5222r-p3.