What is a home agent and what is a foreign agent in mobile networking?

The home agent represents the mobile device in its home network when the device is away; the foreign agent represents it in the visited network.

Home agent:

• An entity in the home network that performs mobility functions on behalf of the mobile device
• Knows the device's permanent home address
• Receives all traffic addressed to the mobile device
• Forwards traffic to wherever the device currently is (via the foreign agent)
• Think of it as a mail forwarding service at your permanent address

Foreign agent:

• An entity in the visited network that performs mobility functions for the roaming device
• Assigns the device a care-of address in the visited network
• Receives forwarded traffic from the home agent and delivers it locally to the device
• Think of it as a local representative — like a hotel concierge receiving your forwarded mail

In the generalized mobility model, the home agent is a conceptual role: the home side knows the permanent identity and current reachability information. In LTE/5G-style terminology, the HSS is not literally a packet-forwarding home agent; it is the home-network subscriber database and lookup point that stores identity, contract, service, and location-related state.

Go deeper:

• Mobile IP (Wikipedia) — the protocol that defines home agent, foreign agent, care-of address and tunnelling — the model these mobility terms come from.