Network Setup Help: Connecting 3 Computers to 2 IPs

[NeoDevin]

My ISP gives me 2 IP addresses, but I have 3 computers to connect to the internet. I have a http://www.linksysbycisco.com/US/en/support/WRT310N/download" as well.

My current rig goes from the cable modem, to the switch, to my 2 current computers.

What I want is to connect the new computer directly to the internet using one of my IP addresses, and use the router to share the other IP address between my 2 existing computers. Ideally all 3 of the computers should be able to access each other via LAN, but this is not essential. Is this possible to set up with my current equipment, and if so, how?

Thanks in advance.
Devin

 

[DaleSwanson]

What kind of router and switch do you have? My router has a page that looks like I could use to map what external IPs go to which internal ones. I don't think it's a very standard feature though.

 

[NeoDevin]

I linked to the product pages for the router and switch that I have, click on the words in my post.

 

[Evo]

I've always just advised clients to use NAT.

http://www.cisco.com/en/US/tech/tk648/tk361/tk438/tsd_technology_support_sub-protocol_home.html

http://computer.howstuffworks.com/nat.htm

 

[mgb_phys]

The one that you connect directly - is this so it is available from outside ?

Assuming it is, and assuming the cable company only gives you one physical wire.

Cable box -> ethernet cable -> dumb switch port 1
Dumb switch port 2 -> externally visible computer
Dumb switch port 3 -> router (assuming it is also a NAT - which most home routers are)
NAT port 2 -> computer 2
NAT port 3 -> computer 3

The externally visible computer is vunerable to attacks, make sure it is well secured, patches upto date, unused ports disabled etc.

 

[NeoDevin]

Cable box -> ethernet cable -> dumb switch port 1
Dumb switch port 2 -> externally visible computer
Dumb switch port 3 -> router (assuming it is also a NAT - which most home routers are)
NAT port 2 -> computer 2
NAT port 3 -> computer 3

Thanks, I was thinking about it while working today and figured the same thing, I'll give it a try later tonight, hopefully everything will work properly.

 

[NeoDevin]

Ok, so I decided to do things a little differently than that, and use 1 ip address for all three machines, but I'm running into a bit of trouble that I can't figure out. The set up posted by mgb_phys above works, with the switch between the cable modem and the router. I've tried to switch the router and the switch, so it goes:

Cable modem -> Router (NAT)
NAT port 1 -> Computer 1
NAT port 2 -> Switch
Switch -> Computers 2 and 3

The problem is that I can't access the internet, or even the router setup page from computers 2 or 3. Any idea what the problem could be?

Thanks in advance.

 

[mgb_phys]

That should work - unless the switch is faulty or the router has some weird extra settign to only allow one machine per port.
Can you connect a computer directly to NAT port 2 and test that

 

[NeoDevin]

Connecting a computer directly to NAT port 2 works, and just using the switch by itself works as well.

 

[mgb_phys]

Sounds like the switch is blocking the broadcast from machines 2/3 when they try and get a dynamic address with DHCP.
Is this a smart switch with some traffic management or just a $10 dumb hub - the dumb one might work better!

 

[NeoDevin]

The switch is the one linked to in the OP, http://www.linksysbycisco.com/US/en/products/EZXS55W . I got it for about $50, 3 years ago. The thing that I don't understand, is that the switch doesn't cause problems with the modem acting as a DHCP.

 

[NeoDevin]

Got it working! The problem was that the router auto-detects whether it needs to cross the cable or not, if it's connected to a computer or a modem, but not a switch. I had to connect it to the uplink port on my switch with a straight through cable (or one of the other ports with a crossover cable).

 

[mgb_phys]

well done - often only the first/uplink port on a switch will autodetect (just to save a few pennies)