Unless you have a home network, your computer will be connected something like this:
local network at 192.168.123.1
subnet mask is 255.255.255
-----------
| ------- +--> NAT
| | | |
| | 100 | |
| | | |
| ------- |
| |
| * ===== |
-----------
localhost
Your USB modem (or whatever your ISP gave you to use their Internet services) connects to the global network using Network Address Translation.