[R] replace text by uniqe number

[Ragia .]

Dear group
kindly if i had data frame of 2 columns that has repeated URLS ..and want to replace those urls with ID's for each one so the url will have
unique ID, how  can I do this
thanks in advance
e.g otf  data

pcworld.com	open.itworld.com
pcworld.com	storage.itworld.com
salon.com	images.salon.com
go.theregister.com	theregister.co.uk
techchuck.com	pcworld.com
ecoustics.com	pcworld.com
 	
Ragia