Originally Posted by SmartAlx
Why is that address different for each different network I join?
The same reason as why the address of every pub I visit is different. They're different addresses.
The phone has it's own address - called a MAC address. You don't need to know what that is usually. Each network is its own little discrete entity away from the internet - usually with addresses in the range 192.168.x.x or 10.0.x.x but they can be many things.
The phone talks to the network it joins and asks for an address - so even if you revisit the same hotspot twice, you may not get the same address.
This is simplified a bit but that's basically how it works.