i was reading about "MOBILE IP"..

they said that :
assume initially we were connected to your home network and then you moved to another network...there is a problem that the person who was communicating with you will be using your old ip address ( that of you home network)...however you are no longer in your home network...

my question is :
where is the problem !?!! lets say im talkin to my friend on msn..and she changed her network..i still can talk to her because when the transport layer establishes a connection with her again,,it will be using her new ip address !
why would we need to keep the old ip address :S i dont get it..could anybody please help me out here. thx

MSN isn't dependent on an IP address because the Microsoft has a server directing traffic. There are other scenarios and protocols where this isn't the case. If you connect directly to the other system it wont be the same, for example:

A friend publishes a folder on their laptop for you to access (perhaps through FTP). You get a link that looks like ftp://123.456.78.910/pics. This link will work but if they take their laptop to work their IP will be different and the link wont work.

It can also be an issue with firewall settings. I have several friends who I regularly make complex connections with, I have their IPs set as trusted in my firewall. If they tried to establish one of those connections after moving their computer to a new IP my firewall would stop them.

The most common scenario where this would be a problem is probably remote desktop. Imagine you have a laptop with remote desktop enabled so you can access it when you're not home. You'd get to it by putting your IP address into your RDC client. When your laptop is at home all you have to do is enter the IP address (123.456.78.910) and you connect to it, but if you left the laptop at work and wanted to access it from home the IP would be different and you wouldn't be able to access it without the new address.

I hope these examples cleared the matter up for you and answered you're questions.