Home network expansion issues

Question

I have a simple home network: ISP cable goes into router (netgear wndr3700 running ddwrt) which provides connection to all devices, static wired clients and dhcp wireless, (see scheme below, left side). Router-ISP connection authentication is based on MAC-ip pair - no passwords or whatever. I need to add another pc to my network; there are some limitations though - it is not physically possible to connect that pc to the router, but possible in front of router. There is a rule: all pcs must be able to reach each other (not required to be in the same subnet, ie pc1 is 192.168.1.2 and pc3 is 192.100.200.300). Since that would be a pc not suited to pass all traffic from and to the router, I decided to split my network by putting a device between the ISP and router (see middle part of the scheme).

Here is the question - what kind of device do I need, another router or just a switch, such as D-link DGS-1100-05 (http://www.dlink.com/uk/en/products/dgs-1100-pd-series-gigabit-smart-switches)? It seems that switch in question is working on Layer 2 and incapable of assigning ips to clients - it would just join my router and pc3 into a network so they would be able to communicate. I am not sure if pcs behind the router (pc1,pc2) will be able to communicate with pc3 through router and switch.

There is another solution - place router in front of everything, put switch in place of router (scheme part 3). Still not sure about communication between clients.

 internet          +    internet                 +    internet
    |              |        |                    |        |
    |              |        |                    |        |
    |              |        |                    |        |
    |              |   +----v---+      +-----+   |   +----v---+      +-----+
    |              |   |        |      |     |   |   |        |      |     |
    |              |   | switch +------> pc3 |   |   | router +------> pc3 |
    |              |   |        |      |     |   |   |        |      |     |
    |              |   +---+----+      +-----+   |   +---+----+      +-----+
    |              |       |                     |       |
    |              |       |                     |       |
    |              |       |                     |       |
+---v----+         |   +---v----+                |   +---v----+
|        |         |   |        |                |   |        |
| router |         |   | router |                |   | switch |
|        |         |   |        |                |   |        |
+-+---+--+         |   +-+---+--+                |   +-+---+--+
  |   |            |     |   |                   |     |   |
  |   +----+       |     |   +----+              |     |   +----+
  |        |       |     |        |              |     |        |
+-v---+  +-v---+   |   +-v---+  +-v---+          |   +-v---+  +-v---+
|     |  |     |   |   |     |  |     |          |   |     |  |     |
| pc1 |  | pc2 |   |   | pc1 |  | pc2 |          |   | pc1 |  | pc2 |
|     |  |     |   |   |     |  |     |          |   |     |  |     |
+-----+  +-----+   +   +-----+  +-----+          +   +-----+  +-----+

I am pretty sure there is an elegant solution that could solve this problem with a switch, but I've never worked with switches before: their definition is pretty straightforward, but lack of experience with them stops me.

Update: third variant works just as intended, pcs can see each other, no issues.

Answer 1

A switch or even a hub is enough.

A router with multiple LAN ports is in fact router+switch or router+hub tandem. E.g. OS of my router sees all LAN ports as a single device (eth1); they are split later on the hardware level. Therefore adding a switch or hub as in the right picture is equivalent to having additional LAN port(s) in the router. All three PCs should behave as if they were connected to LAN ports of the router, unless your switch is manageable and you mangle with its configuration and do something funny, e.g. disable some ports (D-link DGS-1100-05 from your link is a smart switch, it has some basic configuration options).

---

Adding a switch or hub as in the center picture will expose your PC3 to your ISP. PC3 may try to obtain IP address or so, but the ISP will probably reject all communications from it because its MAC will indicate this is a stray (alien) device. It all depends on the ISP's policy and details of your subscription. Home users are usually offered one IP address (regardless whether it's a globally routable IP or one behind a Carrier Grade NAT); if they want to connect more than one device, they have to use NAT (as your router probably does).

However there's no way the ISP could prevent your router from communicating with PC3 via switch or hub in this case. With "proper" manual configuration (rather beyond capabilities of a standard home router firmware, but with like OpenWRT) you could obtain communication between all the three PSc.

If you managed to do this, PC3 would be a "guerrilla" device inside ISP's network. Also note if you used a hub in this scenario, then any communication between your router and PC3 would leak towards the ISP; this is not the case with a switch.

There are more issues, security concerns. This setup is not worth the hassle, unless you have a really good reason.

You could probably talk to your ISP and register additional device (for additional price maybe), but if you just want to add a PC to the network, stick to the right picture. The right one is the right one. :)