VM dynamic IP, yet TBB Ping Monitor sees same IP...

Hi,
can anyone explain why my Asus RT-N56U wireless router gives a different (but always the same) public IP address (for example the IP that TBB Ping Monitor registers) to that which is displayed when I disconnect my router?

I am on Virgin Media 10MB cable, and connect my PC via Cat5 to the router, then Cat5 to the cable modem. Nothing else.

The router always gives out the same IP address (86.21.124.xx) regardless of what Virgin Media renew my actual IP address to...

This is great from the point of view that I can use the TBB Ping Monitor 24/7! but also slightly confusing...
Sam

Dynamic IP's are issued based around leases, and with many providers if you reconnect inside a period of 24 hours you get the same IP address

Often called a sticky IP as a way of explaining it

thanks, I had heard that, but it still doesn't explain why the public IP displayed by my router is different to that displayed by my cable modem (without the router connected)...

Because cable services tend to assign the IP based on the MAC of the network interface presented to it.

So when your router is connected it sees one MAC, and I assume you connect a computer directly to see what IP the modem has, in which case it is really handing it to the computer.

Thanks, that all makes sense.

It's still a bit odd why my IP seems to change more often with the router disconnected, yet it's always the same IP given with the router connected...it may just be coincidence that the IP renews have occurred when I've been testing without the router...time will tell if my router's IP ever changes!