apache 2.2 - Ubuntu - How can I see what an IP is doing in my server?

I'm having problems with my server, and have been using iftop to show connections to and from the server in real time.

It shows an IP address that is constantly connected to/from me, but I can't find any information about what it is used for.

How can I find out what exactly that ip is doing with my server?

EDIT: with help of the answers, I was able to see with iftop the following

my.ip.address:46414 => 199.16.156.20:https

Then with netstat -a I see the following

tcp        0      0 my.ip.address:46414 199.16.156.20:https     ESTABLISHED

I do have an apache web server, but that IP is not showing on the logs. And furthermore, why is it connecting to port 46414?? What is he doing!

Thanks

EDIT2: Ok, thanks to the answer of Daniel t. I'm getting closer. I tried with lsof -i:46475 and this is the output

COMMAND   PID     USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
apache2 22003 www-data   19u  IPv4 716074      0t0  TCP ns.arg2.wirall.com:46475

So it looks like apache is doing something... but as it is an outbound connection, how can I know what is it really doing??