Quote:
Originally Posted by Chris W
That is a standard feature of dynamic ip addressing
would go into the technical details but it's late and i've got another 16 hour day ahead of me tomorrow 
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OK since Chris can't be bothered

It works a little like this (A basic explanation anyway).
When you connect to a network, your computer sends out an IP Address request to the DHCP server of the network, firstly the DHCP server checks your Network Interface Card's MAC (Media Access Control) address against its list of currently leased IPs, then performs one of 2 scenarios.
Scenario 1 - The MAC Address was not found on the list of leased IPs
The DHCP responds to the computer's request with an IP address to use, plus maybe some other information such as DNS servers, Default Gateway, etc, and so on and adds the Computer's MAC address to the list of leased computers.
Scenario 2 - The MAC Address is still leased
The DHCP server sends the still leased IP address to the device that requested it along with other information such as DNS servers, Default Gateway, etc.
On the DHCP server, there are several settings, the most important being Lease Time, this so-called Lease Time is the ammount of time the device connected to the network is allowed to use the IP Address for. If the device does not request a renewal on the lease before the end of the Lease-Time (IE probably turned off or disconnected), then the IP Address is released and can be assigned to another device.
Umm thats pretty much it covered, hope this helps someone