[kwlug-disc] Web references on issues of router behind router?

[unsolicited]

I'm playing with a (different) wi-fi residential router behind my main 
home gateway.

I'm intentionally using the wan port, examining what's what. The WAN 
port is getting DHCP internally, and computers on the subnet get out 
to the internet just fine.

Anyone got any good links to articles that discuss this setup - I'm 
not getting expected results, and I suspect I'm over-thinking things.

e.g.
- I'd be surprised if something like VPN made it through both routers 
to the additional internal subnet. Anyone know of articles that 
discuss what else might be broken?

- as expected, machines on the internal subnets can't talk to each 
other. But what if I wanted them to?

Anyone have any favourite web links that discuss this?

TIA.