Mobile IPv4

The Players

There are 4 entities in Mobile IPv4:

  1. Mobile Node (MN) - Your phone or laptop. The device that moves around.

  2. Correspondent Node (CN) - The server or person you’re talking to. They don’t know or care that you’re mobile.

  3. Home Agent (HA) - A router in your home network. It knows your permanent Home Address and tracks where you currently are.

  4. Foreign Agent (FA) - A router in the network you’ve moved to. It helps your device get a temporary Care-of Address.

The Process

Step 1: You Leave Home

You move from your home network to a foreign network. Like going from home WiFi to a coffee shop.

Step 2: Discovery

The Foreign Agent broadcasts “I’m here!” messages. Your phone hears this and realizes it’s in a new network.

Step 3: Registration

Your phone tells the Home Agent:

“Hey, I’ve moved. My new Care-of Address is X. Forward my packets there.”

The Home Agent updates its records.

Step 4: Packet Flow

When someone sends you a packet:

  • CN sends packet to your Home Address (they don’t know you moved)
  • Home Agent receives it, wraps it in a new packet addressed to your Care-of Address (tunneling)
  • Foreign Agent receives the tunneled packet, unwraps it, delivers to your phone

Step 5: Reply

When you reply, your packet goes directly to CN. No tunneling needed for outbound.

Why Foreign Agent?

You might ask: “Why do we need a Foreign Agent? Can’t the phone just get an IP directly?”

Two reasons:

  • Saves addresses - Multiple mobile devices can share one Care-of Address (the FA’s address)
  • Less work for phone - The FA handles the decapsulation, not your battery-limited phone

The Triangle Problem

Notice the inefficiency?

Packets to you take a detour through Home Agent. Packets from you go direct.

This is called triangle routing. It’s wasteful but simple.

Mobile IPv4 trades efficiency for simplicity. The CN never needs to know you’re mobile.