Ubuntu automatically defaults your network interfaces to using DHCP because it is the simplest way of setting up a network interface. With dynamic, or DHCP-assigned IP addressing schemes for your NIC, the broadcast address is set at
dhclient, the DHCP client used for IP configuration, is initially unaware of where the DHCP server is located, so the request must travel every network until a server replies.
You can find the instruction to use DHCP for your NIC
/etc/network/interfaces, with a line that says
Other settings specific to obtaining DHCP settings are saved in the file named
dhclient.conf under the
/etc/dhcp3/dhclient.conf directory and are documented in the