Ethernet card not enabled at Boot
Some installations of Ubuntu, particularly server installs, may leave the network card inactive (not working) upon boot up.
If your cannot connect, check to see if your network card is activated by typing ip addr.
You should see something like
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue state UNKNOWN link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 inet 127.0.0.1/8 brd 127.255.255.255 scope host lo inet6 ::1/128 scope host valid_lft forever preferred_lft forever 2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000 link/ether 00:b0:d0:07:17:6b brd ff:ff:ff:ff:ff:ff inet 192.168.1.203/24 brd 192.168.1.255 scope global eth0 inet6 fe80::2b0:d0ff:fe07:176b/64 scope link valid_lft forever preferred_lft forever
However, if there is no inet line for eth0, then try sudo ifup eth0.
If that worked, (check with ping -c 2 ubuntu.com) great, but you don't want to have to do that every time you reboot the machine. Check your /etc/network/interfaces file. Using your favorite text editor, edit the file so that the line (you may have to add it)
comes at some point before
iface eth0 inet static address 192.168.1.203 netmask 255.255.255.0 network 192.168.1.0 broadcast 192.168.1.255
(your numbers may differ).
Save, reboot, and recheck. Did that work?
For reference, my very typical fixed-IP single-Network-Card interfaces file looks like this:
# This file describes the network interfaces available on your system # and how to activate them. For more information, see interfaces(5). # The loopback network interface auto lo iface lo inet loopback # This is a list of hotpluggable network interfaces. # They will be activated automatically by the hotplug subsystem. auto eth0 # The primary network interface iface eth0 inet static address 192.168.1.201 netmask 255.255.255.0 network 192.168.1.0 broadcast 192.168.1.255 gateway 192.168.1.1 # dns-* options are implemented by the resolvconf package, if installed dns-nameservers 188.8.131.52 184.108.40.206