Sample

In this case we setup a default route, which is where data packets will be sent if the router cannot determine the next hop:

california> enable
california# config t
california(config)# interface loopback 0
california(config-if)# ip address 1.2.3.4 255.255.255.0

Example

R1(config)#int loopback ?
  <0-2147483647>  Loopback interface number

R1(config)#int loopback
% Incomplete command.

R1(config)#int loopback 0
R1(config-if)#?
Interface configuration commands:
  access-expression       Build a bridge boolean access expression
  arp                     Set arp type (arpa, probe, snap) or timeout
  backup                  Modify backup parameters
  bandwidth               Set bandwidth informational parameter
  bgp-policy              Apply policy propagated by bgp community string
  bridge-group            Transparent bridging interface parameters
  carrier-delay           Specify delay for interface transitions
  cdp                     CDP interface subcommands
  dampening               Enable event dampening
  default                 Set a command to its defaults
  delay                   Specify interface throughput delay
  description             Interface specific description
  diffserv                diffserv (Provisioning)
  exit                    Exit from interface configuration mode
  help                    Description of the interactive help system
  hold-queue              Set hold queue depth
  ip                      Interface Internet Protocol config commands
  keepalive               Enable keepalive
  llc2                    LLC2 Interface Subcommands
  load-interval           Specify interval for load calculation for an
                          interface
  logging                 Configure logging for interface
  max-reserved-bandwidth  Maximum Reservable Bandwidth on an Interface
  mtu                     Set the interface Maximum Transmission Unit (MTU)
  netbios                 Use a defined NETBIOS access list or enable
                          name-caching
  no                      Negate a command or set its defaults
  ntp                     Configure NTP
  rate-limit              Rate Limit
  roles                   Specify roles (by entering roles mode)
  service-policy          Configure QoS Service Policy
  shutdown                Shutdown the selected interface
  snapshot                Configure snapshot support on the interface
  snmp                    Modify SNMP interface parameters
  timeout                 Define timeout values for this interface
  transmit-interface      Assign a transmit interface to a receive-only
                          interface
  trunk-group             Configure interface to be in a trunk group
  tx-ring-limit           Configure PA level transmit ring limit

R1(config-if)#ip address 1.2.3.4 255.255.255.0