After addition of a new network interface card (NIC) in a DHCP client and/or changing cable from one port to another, client might not establish the network connectivity. In such cases first check whether the active NIC's MAC address is there on DHCP server's database. If it is not there then add the MAC address and restart the server. If the problem persists, refresh the DHCP client as below:
# service netif restart
# dhclient eth1
