unable to connect to windows xp by typing in an IP address

Hello,
I am trying to connect to my desktop from my lapton via RDC. I can do it at
home but if i try to type in a IP address in the laptop. it says it cannot
establish a connection. this is the one thing that has been hanging me up. I
can see my desktop files from my laptop i just cant RDP using an ip address.
Any suggestions you may have would be appreciated.
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Thanks