How to update

Hi all,

i have a simple problem with an sql-update.
Lets say i have a table foo which contains a
ip::inet and to counters inet and local.
And i have a table bar which contains
ip::inet inet and local.

I need these two tables for traffic-accounting.
The table foo should contain the sum of traffic of bar,
therefore once a month i would like to run a script which 
performs a query that
selects sum(inet),sum(local) from bar
and
updates foo with these two values where bar.ip = foo.ip.

Who can explain how to perform this action with one query.
I tried this, but it update always one row in foo.

update foo set inet=sum(bar.inet),local=sum(bar.local) where foo.ip = bar.ip;

Thanks in advance

P.S.: Sorry for my bad english, hope you understand what i mean :)