In case your server is connected to a network which broadcasts DHCP you didn't get a chance to configure a static IP address for your server.

Enter ifconfig and copy the output somewhere so you have access to network information you got from a DHCP server.

Open up /etc/network/interfaces with nano and replace

auto eth0
iface eth0 inet dhcp

with

iface eth0 inet static
      address YOUR-IP
      netmask YOUR-NETMASK
      network YOUR-NETWORK
      broadcast YOUR-BROADCAST
      gateway YOUR-GATEWAY
      # dns-* options are implemented by the resolvconf package, if installed
      dns-nameservers YOUR-DNS-SERVERS-SEPARATED-BY-SPACE
      dns-search YOUR-DOMAIN

Reboot your server, check that your internet connection is working (try to ping google.com or some other host) and if everything is ok you can remove dhcp3-* packages from your system.

apt-get --purge remove dhcp3-common dhcp3-client

Note that option –purge removes configuration files of the packages we are removing. If you do not use –purge they will be left on your system.