📡 Lab 6 & 7 — Internetworking I & II · Cheat Sheet

ITS352/DES352 · SIIT · Thammasat University

1 · Connecting Devices — Layer & Role Summary

Key rule: Switch = same network (same subnet). Router = different networks (different subnets).
e.g. 192.168.1.x devices connect through a switch. To reach 192.168.2.x you need a router.

2 · IPv4 Addressing & CIDR

Format: a.b.c.d/n — address is 32 bits = Network prefix (n bits) + Host part (32−n bits)

Formula: Usable Hosts = 2^(32−n) − 2

Special Addresses

3 · How to Calculate Network Address (AND method)

Network Address = IP AND Subnet Mask (bitwise AND operation)

Example: 192.168.10.100/26 → subnet mask 255.255.255.192

IP:   192.168.10.100  =  11000000.10101000.00001010.01100100
Mask: 255.255.255.192 =  11111111.11111111.11111111.11000000
AND:                  =  11000000.10101000.00001010.01000000
                      =  192.168.10.64   ← Network Address

So for 192.168.10.100/26:

• Network:  192.168.10.64
• First host: 192.168.10.65 ✅ (quiz answer)
• Last host:  192.168.10.126
• Broadcast:  192.168.10.127

Example: 112.1.1.194 mask 255.255.255.224 → CIDR?

255.255.255.224 = 11111111.11111111.11111111.11100000
Count the 1s → 27 ones → /27
So: 112.1.1.194/27

Example: 203.159.10.15/19 → Broadcast?

/19 → mask = 255.255.224.0
IP:   203.159.10.15  = 11001011.10011111.00001010.00001111
Mask: 255.255.224.0  = 11111111.11111111.11100000.00000000
AND:                 = 11001011.10011111.00000000.00000000
                     = 203.159.0.0  ← Network Address

Broadcast = Network + flip all host bits to 1:
           = 11001011.10011111.00011111.11111111
           = 203.159.31.255  ✅

4 · Linux Commands (Complete Reference)

Interface Setup

sudo ifconfig eth1 192.168.10.1/24    # Set IP + subnet mask on eth1
sudo ifconfig eth0 down               # Disable interface eth0
ifconfig                              # View all interfaces & IPs

Routing Table

route -n                                          # View routing table (numeric, no DNS lookup)
sudo route add default gw 192.168.10.1            # Add default gateway (for PCs/hosts)
sudo route del default                            # Delete default gateway
sudo route add -net 192.168.30.0/24 gw 192.168.20.2  # Add static route (for routers)
sudo route del -net 192.168.30.0/24               # Delete static route

IP Forwarding (Real Linux PC router only!)

sudo sysctl -a | grep ip_forward      # Check current status → 0=disabled, 1=enabled
sudo sysctl -w net.ipv4.ip_forward=1  # Enable IP forwarding
sudo iptables -P FORWARD ACCEPT       # Allow firewall to forward packets

⚠️ Real Linux PC (physical lab): both commands required on every router PC.
✅ IMUNES: IP forwarding is already enabled by default on router nodes — you do NOT need to run these commands in IMUNES.

Connectivity

ping 192.168.30.8         # Continuous ping
ping -c 5 192.168.30.8    # Send exactly 5 packets → expect "0% packet loss"

5 · Why ip_forward? (The Big Picture)

By default, a Linux PC = end host: it only processes packets addressed to itself — any packet for another IP is silently dropped.

When we use a real Linux PC as a router, we must flip the forwarding flag:

ip_forward = 0  →  "I only care about packets for ME"  (normal PC)
ip_forward = 1  →  "I will also forward packets for OTHER IPs"  (router behaviour)

Why does the lab use PCs instead of real routers?
To teach you that routing tables aren't magic — you must manually add every route yourself. Real routers run protocols (OSPF, BGP) that do this automatically. Here, you learn the manual process.

6 · Routing Table Logic

PC / End Host — only needs default gateway

Destination    Gateway          Genmask          Flags  Iface
0.0.0.0        192.168.10.1     0.0.0.0          UG     eth1   ← default (catch-all)
192.168.10.0   0.0.0.0          255.255.255.0    U      eth1   ← local (auto-added)

• Local network entry (U) → added automatically when you run ifconfig eth1 IP/mask
• Default gateway (UG) → must be added manually with route add default gw

Router — needs a route for every network

Destination      Gateway          Genmask            Flags  Iface
192.168.10.0     0.0.0.0          255.255.255.0      U      eth1   ← direct (auto)
192.168.20.0     0.0.0.0          255.255.255.0      U      eth2   ← direct (auto)
192.168.30.0     192.168.20.2     255.255.255.0      UG     eth2   ← static (manual!)

• Directly connected networks → auto-added when you set the IP
• Remote networks → must add manually: route add -net <network> gw <next-hop-router>

Routing Table Flags

7 · IMUNES — How to Set Up Networks

IMUNES is a network emulator where you drag-drop virtual nodes and configure them.

Basic Workflow

1. Add nodes: drag Router / PC / Hub / Switch from the toolbar
2. Connect nodes: use the Link tool — click source node, then destination node
3. Start experiment: click the green ▶ Play button — nodes become live
4. Open terminal: right-click a node → Shell (or Terminal)
5. Configure using the same Linux commands as real machines

Key IMUNES Tips

IMUNES Setup Order (always follow this)

1. Draw logical diagram (on paper) — identify all IPs first
2. Build topology in IMUNES (add nodes + links)
3. Start experiment (▶)
4. On each node: sudo ifconfig eth0 down (if unused)
5. Set IPs:  sudo ifconfig eth1 <IP>/<prefix>
6. Add routes: default gw (PCs) or static routes (routers)
   [ip_forward NOT needed — IMUNES routers forward by default]
7. Verify: ping -c 5 <destination>

In IMUNES, eth0 is often the management interface — always disable it (ifconfig eth0 down) before setting up lab interfaces, or it may interfere with routing.

8 · Assignment Topologies

Lab 6 — Assignment 3: Two Networks, One Router

[PC A]──eth1──[R1 eth1]──[R1 eth2]──eth1──[PC C]
172.16.101.201/29  .202/29   .13/30    192.168.216.14/30

# PC A
sudo ifconfig eth0 down && sudo ifconfig eth1 172.16.101.201/29
sudo route add default gw 172.16.101.202
 
# PC C
sudo ifconfig eth0 down && sudo ifconfig eth1 192.168.216.14/30
sudo route add default gw 192.168.216.13
 
# Router R1
sudo ifconfig eth0 down
sudo ifconfig eth1 172.16.101.202/29
sudo ifconfig eth2 192.168.216.13/30
# ip_forward NOT needed in IMUNES (already enabled)
# (no static routes needed — R1 is directly connected to both networks)

Lab 7 — Assignment 1: Three Networks, Two Routers

[PC A]──[R1]──[R2]──[PC B]
         Network B (transit)

# PC A
sudo ifconfig eth0 down && sudo ifconfig eth1 192.168.10.8/24
sudo route add default gw 192.168.10.1
 
# PC B
sudo ifconfig eth0 down && sudo ifconfig eth1 192.168.30.8/24
sudo route add default gw 192.168.30.1
 
# Router R1
sudo ifconfig eth0 down
sudo ifconfig eth1 192.168.10.1/24 && sudo ifconfig eth2 192.168.20.1/24
# ip_forward NOT needed in IMUNES (already enabled)
sudo route add -net 192.168.30.0/24 gw 192.168.20.2   # ← manual: R1 can't see Net C directly
 
# Router R2
sudo ifconfig eth0 down
sudo ifconfig eth1 192.168.30.1/24 && sudo ifconfig eth2 192.168.20.2/24
# ip_forward NOT needed in IMUNES (already enabled)
sudo route add -net 192.168.10.0/24 gw 192.168.20.1   # ← manual: R2 can't see Net A directly

Routing table logic for R1:

Lab 7 — Assignment 2: Seven Networks, Four Routers

A(172.16.101.200/29)──R1──C(192.168.66.0/24)──R2──D(192.168.88.0/24)──R3──E(68.66.11.80/29)──R4──G(16.18.11.64/29)
B(192.168.216.12/30)─┘                                                       └──F(211.60.36.16/28)

Key routing rule: Each router needs static routes for every network it cannot directly reach, forwarding toward the next router in that direction.

At R2 — which networks need static routes? → A, B, E, F, G (quiz answer)

9 · Quiz Question Patterns (Labs 6 & 7)

Routing Table Explanation (2-point answer template)

a) The routing table tells a router/PC where to forward each packet based on destination IP.
b) Longest-prefix match: if 192.168.1.50 matches both /24 and /16, the /24 entry wins (more specific).
c) Default route 0.0.0.0/0 (flag UG) matches any destination with no more-specific entry — last resort gateway.
d) Linux command: route -n (the -n flag shows numeric IPs, no DNS lookup — faster and clearer).

10 · Common Mistakes to Avoid