When either machine is connected to the internet and working ok what ip address does it use? E.g on the mac under system preferences what does it say for network->airport. On the pc if you run the command prompt what does it give for "ipconfig"?
The reason is that the easy way to get both machines on the net at the same time is that they share the same outside (WAN) ip address of the Virgin cable modem by doing NAT (network address translation). This would give a connected pc one of the local ip addresses (usually something like 192.168.xxx.xxx). A DHCP server would automatically assign this to your pc but is not necessary. Eg Most of our machines here don't use our DHCP server for their configuration.
If your Virgin modem does not do NAT (because it is just a modem and not a modem/router) then you can either
- buy a NAT modem/router instead
- add a NAT router to it (eg. Apple's Airport Extreme will connect to DSL and do NAT and the wireless access at the same time - and DHCP serving if you want that too)
- effectively get one of the connected PCs to do NAT by "sharing their internet connection". You would have to have that pc on for the other to work and you would also have to wire one of the connections because you cant share wireless to wireless for example.
- a final way is to get Virgin to give you 2 WAN ip addresses if they can although this really isnt the way to go at all.
It is better security-wise to be connected by NAT because then any shared services (eg file sharing) are on non-standard ports for people on the outside of your home.
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