An "Internal IP" is the IP Address that your router, DLS modem or cable modem asigns your computer. If in your network you have a "DHCP server", your computer may change its Ip address from time to time. This behavior is called "Dynamic IP".
The other alternative is to have a "Static IP". This means that your computer will keep the same IP address unless some bad happens to your computer, or you chage some settings on your computer.

I will show you some ways of finding out your Internal or Local IP address.
Windows NT, Windows 2000, Windows XP and Windows 2003:

Click on "Start > Run" and in the blank box type (without quotes) "CMD" amd press the Enter key.

A black window will appear, this is the "Command Prompt"
There, type (without quotes) "IPCONFIG" and press the Enter key. That will show you your Internal IP Address. If you see many entries here, look for the one that is under the name of "Ethernet adapter Local Area Connection:"
In most networks, this number may look like this:
192.168.1.XXX or 192.168.0.XXX (XXX represents any number)

Windows IP Configuration

Ethernet adapter Local Area Connection:

Connection-specific DNS Suffix . :
IP Address. . . . . . . . . . . . : 192.168.0.101
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . : 192.168.0.1

PPP adapter Impact Computers:

Connection-specific DNS Suffix . :
IP Address. . . . . . . . . . . . : 192.168.1.129
Subnet Mask . . . . . . . . . . . : 255.255.255.255
Default Gateway . . . . . . . . . : 192.168.1.129

The "PPP adapter Impact Computers" is because I'm connected to my office through a VPN.
I'll write an article about this later.