calculation of crc-16 in C

I am using an ethernet and for sending ECHO reply,its important to calculate CRC.Should Iconsider data while calculating 16-bit CRC, and if YES then where shoul i append the EXTRA zeros,
1. at the place of CRC itself or
2.in the end..

for example:
if msg is:
45 00 00 3c 02 13 00 00 80 01 [two bytes of crc] [source IP] [destination IP]

should i consider source and destination IP's while calculating CRC-16 or not..?

and where should i append zeroes..?

please,help me..

re: calculation of crc-16 in C

I think that depends on a few different things. What is this going to be used for? Is this a homework question where you're just working on theory, or are you going to create an algorithm and small program to calculate this? If so, what language, and what specifically are you going to be calculating?

re: calculation of crc-16 in C

I am going to create an algorithm,& a small program also..in C'language..
Also ,i am not able to decide polynomial for this calcaulation,could u please help me!!