Configuring IP Address on a Cisco Router

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After you have played a little with some basic commands on your Cisco router you probably want to take it to the next level and connect it to a network so the real fun can begin! Now to be able to TELENT into the router or even just PING it you have to configure at least one of the interfaces with an IP address. Preferably the one you want to plug the cable into…

required steps

If it’s a serial interface check out the Configuring Serial Interfaces tutorial as the steps are slightly different. For ethernet interfaces you only need to do two things:
1. Assigning an IP address and subnet mask
2. Enabling the interface(bringing it up).

If you have a DHCP server running on your network the router might gets an IP Address from the server before you can configure it manually.

optional commands

It’s always good practice to hard-code the speed and duplex and to give the interface some sort of description!


First check which interfaces are available on the router.

R1#show ip interface brief
Interface          IP-Address   OK? Method Status                Protocol
FastEthernet0/0    unassigned   YES unset  administratively down down
Serial0/0          unassigned   YES unset  administratively down down

Now select the interface and add a description.

R1#conf terminal
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)#interface FastEthernet 0/0
R1(config-if)#description LINK TO LAN

Set interface speed and duplex.

R1(config-if)#speed ?
  10    Force 10 Mbps operation
  100   Force 100 Mbps operation
  auto  Enable AUTO speed configuration

R1(config-if)#speed 100
R1(config-if)#duplex ?
  auto  Enable AUTO duplex configuration
  full  Force full duplex operation
  half  Force half-duplex operation

R1(config-if)#duplex full

And finally set the IP Address and subnet mask.

R1(config-if)#ip address ?
  A.B.C.D  IP address
  dhcp     IP Address negotiated via DHCP
  pool     IP Address autoconfigured from a local DHCP pool

R1(config-if)#ip address ?
  A.B.C.D  IP subnet mask

R1(config-if)#ip address
R1(config-if)#no shutdown
*Mar  1 10:07:08.766: %SYS-5-CONFIG_I: Configured from console by console
*Mar  1 10:07:09.175: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up
*Mar  1 10:07:10.994: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to up

As you can see the link state and the line protocol almost immediately changed to up/up.


Verify status and IP Address

R1#show ip interface brief
Interface          IP-Address   OK? Method Status                Protocol
FastEthernet0/0     YES manual up                    up
Serial0/0          unassigned   YES unset  administratively down down

Verify interface description

R1#show interface description
Interface          Status         Protocol Description
Fa0/0              up             up       LINK TO LAN
Se0/0              admin down     down

Detailed information about the interface

R1#show interfaces FastEthernet 0/0
FastEthernet0/0 is up, line protocol is up
  Hardware is AmdFE, address is 000d.bdf6.39a0 (bia 000d.bdf6.39a0)
  Description: LINK TO LAN
  Internet address is
  MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Full-duplex, 100Mb/s, 100BaseTX/FX
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:02, output 00:00:02, output hang never
  Last clearing of "show interface" counters never
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 1000 bits/sec, 1 packets/sec
  5 minute output rate 0 bits/sec, 0 packets/sec
     948 packets input, 58686 bytes
     Received 944 broadcasts, 0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog
     0 input packets with dribble condition detected
     194 packets output, 18734 bytes, 0 underruns
     0 output errors, 0 collisions, 1 interface resets
     0 babbles, 0 late collision, 0 deferred
     3 lost carrier, 0 no carrier
     0 output buffer failures, 0 output buffers swapped out

Verify using show run

R1#show run | section interface FastEthernet0/0
interface FastEthernet0/0
 description LINK TO LAN
 ip address
 speed 100

The section output modifier requires IOS version 12.3(2)T or higher!
The output modifiers are case sensitive! The command show run | section interface fastethernet0/0 wouldn’t work. Also, mind the spaces!

Another version of show run

R1#show run interface FastEthernet0/0
Building configuration...

Current configuration : 119 bytes
interface FastEthernet0/0
 description LINK TO LAN
 ip address
 speed 100


Of course the ultimate test is ping, so let’s ping something!


Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to, timeout is 2 seconds:
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms

And finally let’s ping from a PC

Microsoft Windows [Version 6.1.7600]
Copyright (c) 2009 Microsoft Corporation.  All rights reserved.


Pinging with 32 bytes of data:
Reply from bytes=32 time=2ms TTL=255
Reply from bytes=32 time=1ms TTL=255
Reply from bytes=32 time=1ms TTL=255
Reply from bytes=32 time=1ms TTL=255

Ping statistics for
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
    Minimum = 1ms, Maximum = 2ms, Average = 1ms


commands explained

Commands used with a brief explanation.

interface FastEthernet 0/0: Selects interface FastEthernet 0/0
description LINK TO LAN: Sets the description on the interface
speed 100: Hard-codes the speed of the interface
duplex full: Hard-codes full duplex operation on the interface
ip address Assigns the IP Address
show ip interface brief: Shows all available interfaces on the router, they IP Address and status
show interface description: Shows the interface description
show interfaces FastEthernet 0/0: Shows a very detailed information about the interface
show run | section interface FastEthernet0/0: Show a section of the running configuration that is relevant to us
show run interface FastEthernet0/0: Show the FastEthernet0/0 section of the running configuration

Sisko Warrior

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