Form Fields : copy value of One field , covert it and then instert into another field (same table )

Form (Table Fields) Fields  = Mac Address ,   Converted Mac Address

Requirement :  The user Inputs Mac Address e-g( 843A4B000000 , 843A5AFFFFFF )

I want to convert this Mac Address into  base10 Number and then insert the value into

the Converted Mac Address Field....

Please tell me how and where can I write the code??