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In Computer Networking, and in particular to WiFi-networks, we learn that a "mobile node" (i.e. any mobile unit that can access a router through WiFi) has a permanent IP-address (the "home address"), and an intermediate address ("Care-of-address") when visiting foreign networks, which gets assigned whenever said mobile node associates with a foreign network/router.

From RFC2002:

"A mobile node is given a long-term IP address on a home network. This home address is administered in the same way as a "permanent" IP address is provided to a stationary host. When away from its home network, a "care-of address" is associated with the mobile node and reflects the mobile node's current point of attachment. The mobile node uses its home address as the source address of all IP datagrams that it sends, except where otherwise described in this document for datagrams sent for certain mobility management functions."

I'm having problems understanding the concept of the home address, and how/when this is assigned to the mobile node. What defines the "home network" and "home address" that the protocol refers to here?

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You would only assign a Home Address to a mobile device if it is running a service that others need to access at a fixed address e.g. a Web Server.

In this case when the device is away from it's Home Network it will be assigned a local address on the foreign network (e.g. via DHCP) it would then inform it's Home Network of this address so packets destined for it's Home Address can be forwarded to it via a GRE tunnel

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    It's also worth noting probably that RFC2002 is about "Mobile IP" which allows mobile devices to move from one subnet to another subnet while keeping the same IP address. The original question makes it sound like this is done for all devices on WiFi networks, when in fact it is actually a comparatively rare thing. – Ben Franske Apr 30 '18 at 15:48
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In the context of WiFi networking, mobile devices don't know they are mobile. They are assigned an address (their "home" address) which is part of a IP subnet (their "home" network).

When the device moves and connects to a different subnet, the local router is the foreign network.

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