The easiest method for a NIC to connect to a LAN is via DHCP (dynamic host confirmation protocol). However, this requires that the LAN has connectivity to a DHCP server.
A Debian system relies on /etc/network/interfaces to indicate how a network interface is connected. In the case that we want eth0 to use DHCP, you should find the following entry in that file:
At any time, you can force a system to release the lease of eth0 from a DHCP server by the command dhclient -r eth0. After that, dhclient eth0 reacquires a new lease.