How to rielize such func.?

[Yao Feng]

  There exist 2 tables in one database:log,url_ip,the TABLE log have one 
column:url varchar(100), and the TABLE url_ip have columns :url 
varchar(100) ,ip varchar(15) refelecting the mapping of ip of url.

I want to replace the url in TABLE log with it's ip address using the 
second TABLE url_ip's values:

UPDATE log set url=url_ip.ip where log.url=url_ip.url;

but it failed. saying"fields url_ip.url not exist..."

How can I do with it? any substitute?