As the topic, there were two computer connect to the only route with IPs: 192.168.1.7(mark as A) and 192.168.1.9[wlan0](mark as B). And there were two ethernet cards hardware installed and I configured the other device's IP as 192.168.1.100[eth0], which directly connects with another machine, whose IP has configured as 192.168.1.200(mark as C).
After I added a new route: sudo route add -host 192.168.1.100 gw 192.168.1.9, I can access the B machine via 192.168.1.100 like 192.168.1.9 perfectly.
However, I want to access machine C. I add a new route: sudo route add -host 192.168.1.200 gw 192.168.1.100, it does not work. So, I try another method, ip forward, I enable ipv4 ip forward first: sudo sysctl net.ipv4.ip_forward=1 and than add entries via iptables: sudo iptables -A FORWARD -i eth0 -o wlan0 -j ACCEPT(i want to enable ping the wlan0 in machine C), iptables -A FORWARD -o eth0 -i wlan0 -m state --state ESTABLISHED,RELATED -j ACCEPT. It doe's not work too.
I still can't access machine C in machine A. Btw, I can't control the router, and also can't connect the three machine to the router directly.

Need help, thanks

It sounds like you have two networks: a wi-fi network with .7 and .9, and a wired network with .100 and .200, but you are trying to make them one subnet 192.168.1.x. Without some serious network-fu you can't have two separate unbridged networks act as a single subnet.


Better would be to either connect them all to the same bridge (usually routers have some wired ports as well for this), or to make two subnets and have machine B act as a router between them. If the wired networks need to get to the outside world through the wi-fi, then B will need to do SNAT as well.

You probably should use B as a DHCP server for C and have C on a different subnet. Then B can be the gateway between subnets.

Any luck getting it to work?

Yes, I don't know how to.
btw, Im sorry for late reply because of COVID-19.

No. A and B connected within a DHCP server router, B and C connected directly without a server. thks