ITIS 3100 – Lab 2
Routers
Overview:
The purpose of this lab is to introduce the student to some of the functions of a router, specifically
bridging communication between two networks. For this lab one router (a Linux based system) will be
use to connect 2 different networks together and allow them to communicate. The router for this lab
will be a low end PC with two NICs installed. The operating system for the router will be Debian
Linux.
This lab may be done individually or by pairs. If the lab is done in pairs, each student must hand in
their own lab report. For pairs each student will “own” one of the networks. They will cooperate to
get the clients to communicate through the router.
The 3rd octet of their IP address will be assigned by the instructor. See web site or the posting in the
lab for your network id. For students working in pairs they will need to work out the address to be
shared between their clients.
Note: all the examples below use an arbitrary network id of 99.
Hardware requirements
Equipment:
 One Linux router (PC) with 2 NICs
 Two Switches
 either a Linksys BEFSX41 or WRT54G
 Important: only the switch function of the Linksys router will be used
 Two Client Linux VMs on separate PCs
 Cat 5e cables
Procedure:
Preliminary
Ensure your environment will support the lab
 The lab will require 3 computers and 2 Linksys routers overall
 Ensure there are enough power outlets and Ethernet cables available
 (5 power ports and 4 cables)
Part 1
The first part of the lab will be setting up two Linux VMs on two separate physical machines. You will
be recycling the VMs that were set up in your last lab. You may use either Debian or CentOS (or one
of each). VMs are being used so you don’t mess up the base machines for other classes (and also don’t
permanently mess up your own VMs). The two VMs will be set up with IP addresses for different
network class environments, one for 172.16 (a Class B network) and one for 192.168 (a Class C
network).
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ITIS 3100 – Lab 2
Routers
After the machines are set up they will be connected together via a switch to show they cannot
communicate since they are in different networks. In Part 2 of this lab they will be connected via a
router and communication will be enabled.
Note: Depending on how Linux was set up when the OS was installed, DHCP may or may not be
enabled for your NIC cards. (DHCP is an automatic IP address assigner). Having DCHP enabled on
either your VM client or on the NIC cards on the router will make the lab impossible to run. One of
the steps you will need to do for each of the VMs and the Router is to ensure DHCP is not enabled.
The following method of checking and, if need be, changing is valid for Debian Linux:
 Locate the interfaces configuration file
o /etc/network/interfaces
 View the contents:
o Look for the iface line e.g.
 iface eth0 inet dhcp
o If it has static at the end its probably ok, if there is the dhcp change it to static
 Use vi, vim or another editor of your choice
 Change dchp to static
o Add lines, if needed, to assign an address and a netmask to the NIC for that iface
 Use man interfaces to find the command(s) to do that
 Save the file and restart the network card(s) as needed
o /etc/init.d/networking restart
Lab steps:
1. Configure VM1s IP environment
 change to root user
■
su (or su -)
 Configure the IP address to a Class B (172.16.99.1 – 172.16.99.253), e.g. 172.16.99.10.
Remember to change the 99 to your own network id. See the IP table on the web page.
■ ifconfig eth0 172.16.99.10
2. Configure VM2s IP environment
 Configure the IP address to a Class C (192.168.99.1 – 192.168.99.253), e.g. 192.168.99.10
■
ifconfig eth0 192.168.99.10
3. Connect the two VMs together and test
 Connect VM1 and VM2 together with a Linksys Router
Use only the switched part (ports 1 and 2) - See
 Figure 1
4. Ping to verify the two VMs cannot communicate
 From VM1
■ ping self, for example
 ping 172.16.99.10
■ ping other
 ping 192.168.99.10
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ITIS 3100 – Lab 2
Routers
From VM2
■ ping self, for example
 ping 192.168.99.10
■ ping other
 ping 172.16.99.10
5. Optional Bonus (+2pts)
 Change the net mask for both VMs to /1
 Repeat the pings and note the results
 Return the net masks to normal

WAN
Linksys
1
2
3
4
PC1
PC2
VM1
VM2
Figure 1
Part 2
Add the Linux router to the network and make necessary changes to the VM IP configurations. The
router and the VM systems will be reconfigured to allow communications between the two disparate
networks.
Lab Steps:
6. Configure the Linux router IP
 Log onto the router
 Userid: student
PW: student
 Change to root user
 su
 root pw: rootpw
 Configure the gateway for 172.16.99.0 network
■ ifconfig eth0 172.16.99.254
 Configure the gateway for 192.16.99.0 network
■ ifconfig eth1 192.168.99.254
 Enable the ip forward function in Linux (very important!)
■ ip forwarding can be enabled for this session by the following command:
 echo 1 > /proc/sys/net/ipv4/ip_forward
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ITIS 3100 – Lab 2
Routers
■
ip forwarding can be permanently enabled by the following:
 find the sysctl.conf file (usually in the /etc directory)
 find and uncomment the following line:
 # net.ipv4.ip_forward = 1
 The actual parameter on the line may vary by Linux distribution, read the
comments to understand which command enables ip forwarding
 Use the ifconfig and route commands to verify
■ ifconfig
■ route
7. Modify VM1s IP
 add default gateway for eth0 (very important!)
■ route add default gw 172.16.99.254
 use route command to verify the routing table
■ route
8. Modify VM2s IP
 add default gateway for eth0 (very important!)
■ route add default gw 192.168.99.254
 use route command to verify the routing table
■ route
9. Physically connect the system together
 The Debian router will be inserted between the two VMs
 See Figure 2
10. Test connections
11. Ping to verify the two VMs can now communicate
Note: you may need to wait a short while (1-2 minutes) for the route tables to establish
connections.
 ping self, for example (just for the fun of it)
■ ping 172.16.99.10
 ping the gateway
■ ping 172.16.99.254
 ping other
■ ping 192.168.99.10
Clean-up
12. Return everything to normal
 Uncable all hardware and reconnect the pc to the wall port
 Put away the Linksys “switches” and power bricks
 Return router to table
Deliverables:
An organized written lab report including:
 Screen shots of the IP configurations for the VMs.
 Screen shots of the results of pings from both Parts 1 and 2
 See the Lab 2 suggested format for ideas
Points to cover:
 Setup of the original network and how the network “worked”
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ITIS 3100 – Lab 2

Routers
Setup of the routed network
o Changes that were made and why they were done
Remember: communication is a critical IT skill! Part of the grade for the lab will be on the written
aspect of the lab as well as the physical details.
Trouble shooting:

Just doesn’t work:
o Double check DHCP is disabled and a static IP is set for all machines
 Can’t change IP:
0. switch to root user
1. use ifconfig to show the basic information
 Can’t ping gateway: make sure to change default gateway after configuring the IP
Use:
 route add default gw 172.16.99.254
- or  route add default gw 192.168.99.254
 Can’t ping other network: make sure to open the Linux IP forward function
Use:
o echo 1 > /proc/sys/net/ipv4/ip_forward
Figure 2: Example Lab 2 network for userid 33.
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ITIS 3100 – Lab 2
Routers
Notes:
Router information
Machine ID: router n
ID: student
PW: student
Root PW: rootpw
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