Inline count on a query

Hi,

Let's say we've got a fairly basic table :

create table networks (
lan_id text not null,
net_id text not null,
port_id text not null
);
create index net_uniq on networks(lan_id,port_id);

The query conundrum I am facing is that I need to add metadata to the output of the query that indicates the count of
portsa given net has on a lan. 

So, for example, given :
insert into networks(lan_id,net_id,port_id) values('L1','N1,'P1');
insert into networks(lan_id,net_id,port_id) values('L1','N1,'P2');

The metadata count would be 1, 2 (because 'N1' has 'P1' and 'P2' on 'L1').

Is there a sensible way to query this without stressing out Postgres too much ?  I'm guessing a CTE of some sort ?

Laura