I have a child router with 192.x.x.x address space. On any machine here I can access the 10.x.x.x address space. The reverse is not true. A machine on 10.0.x.x cannot ping 192.168.x.x. How do I configure my router to make this possible?
It sounds like you have more than one NAT gateway on a small network. Doing “Double NAT” — as this is called — usually causes several hassles, just one of which is the one you’re experiencing: part of your network is on the public/WAN/untrusted side of one of your NATs, and can’t reach the machines on the other side.
The best solution to avoid this and other hassles of Double NAT is to reconfigure the downstream router so it’s not doing NAT. Some routers let you disable NAT and make the WAN port become bridged in with the LAN ports. Others don’t have a way to do that, so you have to just disable the DHCP Server on them, and plug one of their LAN ports into a LAN port on the upstream router.
You will need to make changes on both your routers -
On the child router - Assign the WAN interface a static IP address (you can either assign it on the child router by having it in the 10.x.x.x range but outside the DHCP range, or can use the parent router to do DHCP Reservation to assign an address from the parent router). Disable NAT on the child router.
On the parent router you need to set a static route for 192.168.x.x with a gateway of the child routers WAN IP address.