I've some doubts of IP addresses.

Where are the our IP addresses stored(I mean is it in our service provider's server ?). Is there any way we can change our ip after we have logged in. ??
What is the significance of our IP address.
I use a dialup connection.
One thing I've noted is that the first part of my ip address always remains the same.
(ie. For example,
If my ip address is
Every time I login my ip addrress is 44.23.22.*** )
Why is the beggining part always the same ??

Something more..
What do u mean by adapter address, Subnet Mask.??