Forcing a network card to do a DHCP discover

How do you force a network card to do a DHCP discover i.e. request a new IP address?

I've tried restarting the network and routing but it doesn't seem to work.

Thanking you in advance,
Jonathan.

If you have the card set to DHCP in /etc/rc.conf,
% service netif restart

Otherwise, just re-run dhclient(8). I wouldn't be surprised if HUPping it would do it, but the man page doesn't mention that.

Additional: restarting dhclient just gets a new lease, which will probably have the same IP address as the previous one.