Either use a single subnet throughout your network, or if you subnet than the switch/hub should have information as to how to reach each subnet and back.
When you want to subnet the IP address on that particular PC should be routable, that is from Public IP addresses space. If you are suing private IP addresses than you ADSL router should do NAT to convert private IP to public and vice versa on its return path.
The ADSL router should be the default gateway if you are using a single subnet. If you are using multiple subnets with managed switches than each switch should be the default gateway for its subnet and router should be default gateway for the switches. If you are using unmanaged switches than again router should be the default gateway.
For switch to switch connection or any other similar connection (hub-hub, pc-pc, router-router) you need to use a cross cable, otherwise use a straight cable.
This was first published in May 2003