Day 8 Lab — Router Interface Configuration

Objective

Configure a router to connect three separate subnets and verify that end devices can reach each other across them. This is the foundation of routing — a router with one interface per subnet, each interface acting as the default gateway for that subnet’s hosts.

R1’s three interfaces:

• G0/0 → 15.255.255.254/8 → SW1 → PC1 (15.0.0.1)
• G0/1 → 182.98.255.254/16 → SW2 → PC2 (182.98.0.1)
• G0/2 → 201.191.20.254/24 → SW3 → PC3 (201.191.20.1)

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Step 1 — Set the Hostname

Router> en
Router# config t
Router(config)# hostname R1
R1(config)#

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Step 2 — View Interfaces Before Configuration

Before touching anything, check the current state of R1’s interfaces:

R1(config)# do show ip interface brief

Interface         IP-Address    OK? Method  Status                Protocol
GigabitEthernet0/0  unassigned  YES unset   administratively down down
GigabitEthernet0/1  unassigned  YES unset   administratively down down
GigabitEthernet0/2  unassigned  YES unset   administratively down down
Vlan1               unassigned  YES unset   administratively down down

All three interfaces are unassigned and administratively down — Cisco routers ship with interfaces shut down by default. Nothing will pass traffic until they’re explicitly enabled.

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Step 3 — Configure IP Addresses and Enable Interfaces

Configure each interface with its IP address, a description, and bring it up with no shutdown:

R1(config)# interface gigabitEthernet 0/0
R1(config-if)# ip address 15.255.255.254 255.0.0.0
R1(config-if)# description ## to SW1 ##
R1(config-if)# no shutdown

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

R1(config-if)# int g0/1
R1(config-if)# ip address 182.98.255.254 255.255.0.0
R1(config-if)# description ## to SW2 ##
R1(config-if)# no shutdown

%LINK-5-CHANGED: Interface GigabitEthernet0/1, changed state to up
%LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/1, changed state to up

R1(config-if)# int g0/2
R1(config-if)# ip address 201.191.20.254 255.255.255.0
R1(config-if)# description ## to SW3 ##
R1(config-if)# no shutdown

The %LINK-5-CHANGED and %LINEPROTO-5-UPDOWN messages confirm each interface comes up as soon as no shutdown is entered. Descriptions are optional but good practice — they make the config readable at a glance.

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Step 4 — Verify Interfaces After Configuration

R1# sh ip interface brief

Interface         IP-Address       OK? Method  Status    Protocol
GigabitEthernet0/0  15.255.255.254 YES manual  up        up
GigabitEthernet0/1  182.98.255.254  YES manual  up        up
GigabitEthernet0/2  201.191.20.254  YES manual  up        up
Vlan1               unassigned      YES unset   admin down down

All three interfaces are now up/up with their assigned IPs. The Method column shows manual — meaning the addresses were configured by hand, not via DHCP.

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Step 5 — View Running Config and Save

show run confirms the interface configuration looks correct:

interface GigabitEthernet0/0
 description ## to SW1 ##
 ip address 15.255.255.254 255.0.0.0
 duplex auto
 speed auto
!
interface GigabitEthernet0/1
 description ## to SW2 ##
 ip address 182.98.255.254 255.255.0.0
 duplex auto
 speed auto
!
interface GigabitEthernet0/2
 description ## to SW3 ##
 ip address 201.191.20.254 255.255.255.0
 duplex auto
 speed auto

Then save:

R1# write
Building configuration...
[OK]

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Step 6 — Configure PC IP Addresses

Each PC needs a static IP in its subnet. Done through the Config → FastEthernet0 tab in Packet Tracer:

PC1 — IP: 15.0.0.1 / Mask: 255.0.0.0 / Gateway: 15.255.255.254

PC2 — IP: 182.98.0.1 / Mask: 255.255.0.0 / Gateway: 182.98.255.254

PC3 — IP: 201.191.20.1 / Mask: 255.255.255.0 / Gateway: 201.191.20.254

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Step 7 — Test Connectivity

From PC1, ping PC2 and PC3 to confirm routing is working across all three subnets:

C:\>ping 182.98.0.1

Request timed out.
Reply from 182.98.0.1: bytes=32 time<1ms TTL=127
Reply from 182.98.0.1: bytes=32 time<1ms TTL=127
Reply from 182.98.0.1: bytes=32 time=1ms TTL=127

C:\>ping 201.191.20.1

Request timed out.
Reply from 201.191.20.1: bytes=32 time<1ms TTL=127
Reply from 201.191.20.1: bytes=32 time<1ms TTL=127
Reply from 201.191.20.1: bytes=32 time<1ms TTL=127

Both pings succeed. The first packet times out on each — that’s ARP resolving the MAC address before the ICMP echo can be sent, which is normal. TTL=127 confirms the traffic is passing through one router hop (started at 128, decremented by 1).

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Key Takeaways

• Router interfaces are shut down by default — always need no shutdown to bring them up
• Each interface is its own subnet — the router connects them and forwards traffic between them
• show ip interface brief is the go-to command for a quick status check on all interfaces at once
• Descriptions on interfaces (description ## to SW1 ##) are optional but make configs much easier to read and troubleshoot
• The first ping often times out — ARP has to resolve the destination MAC before ICMP can flow, which takes one packet’s worth of time
• TTL=127 on the replies tells you the traffic crossed exactly one router (128 default TTL - 1 hop = 127)

Resources

• Jeremy’s IT Lab — Day 7 Lecture Part 1
• Jeremy’s IT Lab — Day 8 Lecture Part 2
• Jeremy’s IT Lab — Day 8 Lab