What are the legitimate ways a client's IP address can change even though the user hasn't explicitly changed it?

If I have a UDP server repeatedly receiving requests from the same client machine, when might the client's incoming IP address change even though the user is actually still on the same machine and has not done anything deliberate to change her IP address?

I can think of two cases: (1) packets ar开发者_如何学Ce coming in via proxy servers that use a pool of different IP addresses, and (2) the client machine is using a dynamically assigned IP address that has expired and been reassigned to a new IP address.

Are there any other cases?

Is it a mobile client? If so, it could change networks. For example -

• A phone switching b/w 3G and wi-fi
• A laptop changes wi-fi networks (this is infrequent though)