How to fix routing behind off-the-shelf router?

Question

I have been fighting with a routing problem for a whole evening today, I have finally found what is the symptom, but I have no idea how to fix the problem.

Basically, I have a router that is connected to internet and I have multiple PCs sitting behind it.

• router: 192.168.1.1

• pc-s: 192.168.1.0/24 (if this is the correct syntax)

I want to create a LAN party in this subnet. So that 192.168.1.100 could play with 192.168.1.101.

The problem is, that it doesn't work.

After hours of head scratching, firewall configurations, router configurations and debugging, I finally found the symptom.

I compiled two programs based of MSDN winsock sapmple and launched a server on machine 192.168.1.100

Then I launched the client (192.168.1.101), to connect to server (192.168.1.100). There is a silence.

But when I join the two PC's with a patch cable on hand crafted/static IP network 192.168.3.0/24, alongside the still functional 192.168.1.0/24 network.

So when server is 192.168.3.1 and 192.168.1.100, and client 192.168.3.2 and 192.168.1.101, the connection goes through. Both to 192.168.1.100 and to 192.168.3.1.

I have a Linksys router, and it seems it's swallowing local network traffic. Is there a fix for that?

EDIT: Route table on router has this:

Destination         LAN IP        Subnet Mask       Gateway        Interface
192.168.1.0                       255.255.255.0     0.0.0.0        LAN & Wireless
xxx.xxx.xxx.0                     255.255.252.0     0.0.0.0        WAN (Internet)
0.0.0.0                           0.0.0.0           xxx.xxx.xxx.1  WAN (Internet)

Answer 1

The short answer to your question 'Is there a fix (for my router swallowing local traffic)?' is no, because it is not happening.

The longer answer is when a host sends traffic to another host it will calculate whether the host is on the same subnet (by using the destination address and its own mask) and, if it is, it will send out an arp request for that local host's MAC address, the router does not get involved.

How are you connecting the PCs with the patch cable? Do they have secondary NICs?

I suspect that the reason it is working with the patch cable is because there is some sort of firewall on one of the PCs (not necessarily the target) that is configured for the 192.168.1.0/24 interface but not the new interface you are setting up.