Since MAC address never changes of our computers and IP address of our computer changes as network changes . I'm confused between IP & MAC very much.Data link layer layer needs MAC whereas IP is required by Network layer.
What is all these??

The IP address is used in TCP/IP protocols to identify a device on the network to the network. It is unique only to the network it is connected to. IP addresses are assigned by the server being connected to and managed by that server.

MAC addresses are hardware encoded to the device.

The IP address of your router changes occasionally if you have DSL with a Dynamic address, or if you power cycle the router like unplugging it for a few seconds and turning it back on again (handy if you want to download stuff from a file hosting site that only lets you do so every few hours). You can see the address change if you visit one of the sites that will list your address (seemyip dot org). The computers on the network have IP addresses too and they do not change. Each PC in the house will have a different address the system uses to identify specific ones, usually in a range and differing only by a series of numbers at the end (192.127.200.15 192.127.200.16 etc.). To view the numbers at each PC you can use a 3rd party network info tool or use the command prompt and type "ipconfig \all" with out the quotes (theres a space between the command and the switch \all).