Dynamic Host Configuration Protocol is the default way for connecting up to a network. The implementation varies across Operating Systems, but the simple explanation is that there is a server on the network that hands out IP addresses when requested. Upon connecting to a network, a DHCP request will be sent out from a new member system. The DHCP server will respond and issue an address lease for a varying amount of time. If the system connects to another network, it will be issued a new address by that server but if it re-connects to the original network before the lease is up- it will be re-issued that same address that it had before.

To illustrate this point, say you have your phone set to wifi at your home. It will pick up a DHCP address from your router, before you head to work and connect to your corporate network. It will be issued a new address by your DHCP server before you go to starbucks for your mid-morning coffee where you'll get another address there, then at the local restaurant where you get lunch, then at the grocery store, and so on and so on.