Basic Interface Configuration (IP Addressing)

What Is an Interface?

An interface is a physical or logical connection point on a network device. Think of it as a port where network cables plug in — or in the case of logical interfaces, a virtual connection created in software. Each interface needs its own IP address and subnet mask to participate in Layer 3 communication.

Physical Interfaces

These correspond to actual physical ports on the device:

Logical Interfaces

Logical interfaces do not have a physical port — they are created in software. The most common example on a Cisco switch is an SVI (Switch Virtual Interface), which is used to assign management IP addresses to a switch or to route between VLANs on a Layer 3 switch. For a full inter-VLAN routing walkthrough, see Inter-VLAN Routing on a Layer 3 Switch.

IP Addressing Basics (Quick Review)

Before configuring an interface, you need to understand the two values you will assign: an IP address and a subnet mask. If you need a deeper refresher, see IP Address Classes and Subnetting Guide.

Lab Scenario

In this lab, you will configure IP addressing on the following topology:

  [PC1]──────────────────────────[NetsTuts_R1]──────── WAN
  192.168.1.10/24          Gi0/0: 192.168.1.1/24
                           Gi0/1: 10.0.0.1/30

  [NetsTuts_SW1]
  Vlan1: 192.168.10.1/24
  (management access)
  

Step-by-Step: Configure IP on a Router Interface

Step 1: Enter Privileged EXEC Mode

From User EXEC mode (the > prompt), type enable to enter Privileged EXEC mode (the # prompt). If an enable secret is set, you will be prompted for a password.

NetsTuts_R1>enable
NetsTuts_R1#
  

Step 2: Enter Global Configuration Mode

Global Configuration mode is required before entering any specific interface or feature configuration.

NetsTuts_R1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
NetsTuts_R1(config)#
  

Step 3: Enter Interface Configuration Mode

Specify the exact interface you want to configure. Use the full name or the accepted abbreviation. You can press Tab to auto-complete.

NetsTuts_R1(config)#interface gigabitEthernet0/0
NetsTuts_R1(config-if)#
  

Step 4: Assign IP Address and Subnet Mask

Assign the IP address and subnet mask using the ip address command. Both values must be entered — IOS will reject the command if either is missing.

NetsTuts_R1(config-if)#ip address 192.168.1.1 255.255.255.0
  

Step 5: Enable the Interface (no shutdown)

Router interfaces are administratively down by default — unlike switch ports which are up by default. You must explicitly enable each router interface with no shutdown.

NetsTuts_R1(config-if)#no shutdown
  

IOS immediately responds with system messages confirming the interface came up:

%LINK-5-CHANGED: Interface GigabitEthernet0/0, changed state to up
%LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/0, changed state to up
  

Step 6: Add a Description (Best Practice)

Adding a description to each interface is a professional best practice. It makes the configuration self-documenting and helps during troubleshooting.

NetsTuts_R1(config-if)#description LAN - Connected to NetsTuts_SW1
  

Step 7: Exit and Save

NetsTuts_R1(config-if)#exit
NetsTuts_R1(config)#exit
NetsTuts_R1#
%SYS-5-CONFIG_I: Configured from console by console
NetsTuts_R1#wr
Building configuration...
[OK]
NetsTuts_R1#
  

Configuring a Second Interface (GigabitEthernet0/1)

Most routers have multiple interfaces. Here is how to configure the second interface (Gi0/1) for a WAN or uplink connection in the same session:

NetsTuts_R1#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
NetsTuts_R1(config)#interface gigabitEthernet0/1
NetsTuts_R1(config-if)#description WAN - Uplink
NetsTuts_R1(config-if)#ip address 10.0.0.1 255.255.255.252
NetsTuts_R1(config-if)#no shutdown
NetsTuts_R1(config-if)#exit
NetsTuts_R1(config)#exit
NetsTuts_R1#wr
Building configuration...
[OK]
NetsTuts_R1#
  

Configuring IP on a Layer 3 Switch (SVI)

On a Cisco switch, you cannot assign an IP address to a physical port directly (unless it is a Layer 3 switch in routed mode). Instead, IP addresses are assigned to SVIs (Switch Virtual Interfaces) — logical interfaces associated with a VLAN. The most common use is assigning a management IP to Vlan1 so you can SSH or Telnet into the switch remotely. See SSH Configuration Lab for the full setup.

Example: Assign Management IP to VLAN 1

NetsTuts_SW1>enable
NetsTuts_SW1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
NetsTuts_SW1(config)#interface vlan 1
NetsTuts_SW1(config-if)#description Management Interface
NetsTuts_SW1(config-if)#ip address 192.168.10.1 255.255.255.0
NetsTuts_SW1(config-if)#no shutdown
NetsTuts_SW1(config-if)#exit
NetsTuts_SW1(config)#ip default-gateway 192.168.10.254
NetsTuts_SW1(config)#exit
NetsTuts_SW1#wr
Building configuration...
[OK]
NetsTuts_SW1#
  

Verifying the Configuration

After configuring interfaces, always verify using show commands before testing connectivity. These are the same commands used in real enterprise environments.

1. show ip interface brief

The fastest way to get an overview of all interfaces, their IP addresses, and their status. See Show IP Interface Brief for a full explanation.

NetsTuts_R1#show ip interface brief
Interface              IP-Address      OK? Method Status                Protocol
GigabitEthernet0/0     192.168.1.1     YES manual up                    up
GigabitEthernet0/1     10.0.0.1        YES manual up                    up
  

2. show interfaces gigabitEthernet0/0

Shows detailed Layer 1 and Layer 2 statistics including errors, bandwidth, and duplex. See Show Interfaces for full output explanation.

NetsTuts_R1#show interfaces gigabitEthernet0/0
GigabitEthernet0/0 is up, line protocol is up
  Hardware is CN Gigabit Ethernet, address is 0060.2f3a.1b01 (bia 0060.2f3a.1b01)
  Description: LAN - Connected to NetsTuts_SW1
  Internet address is 192.168.1.1/24
  MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Full-duplex, 1000Mb/s, media type is RJ45
  input errors 0, CRC 0, frame 0, overrun 0, ignored 0
  output errors 0, collisions 0, interface resets 0
  

3. show running-config interface gigabitEthernet0/0

Displays only the configuration lines that apply to this specific interface. See Show Running Config.

NetsTuts_R1#show running-config interface gigabitEthernet0/0
Building configuration...

Current configuration : 98 bytes
!
interface GigabitEthernet0/0
 description LAN - Connected to NetsTuts_SW1
 ip address 192.168.1.1 255.255.255.0
 duplex auto
 speed auto
!
end
  

4. Test Connectivity with ping

Use ping to test whether the interface can reach other devices. A successful ping confirms IP addressing and basic connectivity are correct. For hop-by-hop path verification, also see traceroute.

NetsTuts_R1#ping 192.168.1.10
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.10, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/2 ms
  

Common Mistakes and Troubleshooting

For a structured approach to diagnosing network issues, see Troubleshooting Methodology and Troubleshooting Connectivity.

Interface Status Combinations — What They Mean

These status codes appear in show ip interface brief and show interfaces. Interface state change messages also appear in syslog / show logging output.

Key Points & Exam Tips

• Router interfaces are administratively down by default — always run no shutdown after assigning an IP. Switch physical ports are up by default, but SVIs are down by default.
• The ip address command requires both the IP address and the subnet mask — ip address 192.168.1.1 255.255.255.0.
• To remove an IP address, use no ip address in interface configuration mode.
• show ip interface brief is the fastest verification command — memorize its output columns (Status / Protocol). Use show ip route to verify routing table entries after configuring interfaces.
• Status up / Protocol down = Layer 2 problem (cable, encapsulation). Administratively down = shutdown not reversed.
• On a Layer 2 switch, the SVI (interface vlan 1) is used for management access. Always add ip default-gateway so the switch can be managed from other subnets via SSH or Telnet.
• Always add a description to every interface — it is a professional best practice and helps during troubleshooting.
• Two interfaces that need to communicate directly must be in the same subnet. Mismatched subnets require a router.
• Save with wr after every change — the running-config lives in RAM and is cleared on reload. See Saving & Managing Cisco Configurations.
• Always verify with ping after configuration to confirm end-to-end connectivity.

TEST WHAT YOU LEARNED