COMP3250 – IP Addressing, Subnetting, ARP & ICMP

Teaching Staff

Lecturers
- Rajan S
- Lashmi Kondoth — lashmi.kondoth@mq.edu.au
- Hassan Bajwa — hassan.bajwa@mq.edu.au
- Josh Bhalotia — josh.bhalota@mq.edu.au
- Johnson Rouslie (Junior) — johnsonrouslie.junior@mq.edu.au

Unit Goals & Objectives

Attain advanced understanding of design & implementation of computer networks.
Achieve in-depth mastery of key protocols in the TCP/IP suite.
Engage in hands-on laboratory exercises for practical competence.

hankaran (Unit Convenor)
Office: E6A-337
Ext: $9537$
Email: rajan.shankaran@mq.edu.au
Lashmi Kondoth
Email: lashmi.kondoth@mq.edu.au

Teaching Assistants (TAs)
- Abhijit Ajith Nair (Senior TA) — abhijithajith.nair@mq.edu.au / abhi.nair@mq.edu.au

Learning Outcomes

Communicate advanced networking knowledge (esp. Internet technologies) to a non-specialist audience.
Design TCP/IP-based networks/protocols & integrate them with other technologies.
Produce professional documentation demonstrating practical networking skills.
Identify & analyse security issues in computer networking.
Work independently with high autonomy & accountability.

Topics Covered

Concentration on the TCP/IP protocol stack
- Network-layer design
- Internet addressing schemes
- Routing protocols
- Transport-layer design
- Network security
- Selected advanced topics

Course Material

iLearn site: http://learn.mq.edu.au/
Primary Textbooks
- D. Comer, “TCP/IP Volume One” (6ᵗʰ Ed.)
- A. Tanenbaum, “Computer Networks”
Additional Reading
- Kurose & Ross, “Computer Networking: A Top-Down Approach” (5ᵗʰ Ed.)
- W. Stallings, “Business Data Communications” (5ᵗʰ Ed.)
- P. Oppenheimer, “Top-Down Network Design” (3ʳᵈ Ed.)
RFCs (Requests for Comments)
- Official, authoritative, immutable Internet standards from IETF.
- Example: RFC 791 — Internet Protocol.
- Reading RFCs deepens technical insight; links provided weekly on iLearn.

Assessment Summary

Major Assignment (Individual) — $40\%$
- Problem-solving focus; due 26 Oct 2025.
- Encourages research, critical thinking, and real-world application.
- Late penalty per university policy.
Quiz (On-Campus, Closed Book) — $30\%$
- Week 13 workshop; 2-hour duration; covers all previous lectures.
Lab Book (Individual) — $30\%$
- Select any 6 of 10 workshop labs (Weeks 2–11).
- Exercises target “Apply” & “Analyse” levels of Bloom’s Taxonomy.
Grading Rule: Overall mark $\ge 50\%$ required to pass.
- No hurdle requirements in this unit.

Workshops & Tools

Hardware: Cisco networking equipment.
Software: Wireshark packet analyser.
Refer to official timetable for session details.

Unit Overview – Key Issues

Addressing
Routing
Group Communication
Security
Applications
(Visual slide depicted Autonomous Systems with CIDR blocks such as $1.2.0.0/16$ and $4.5.0.0/16$ .)

Lecture 01 — Introduction to IP Addressing

Purpose & Positioning

First of two recap lectures covering content from COMP2250 & COMP2270.
Focus today: IP Addressing.
Next lecture: IP Header structure & fields.

Why Addressing Matters

Internet = interconnection of diverse physical networks.
Packets move across heterogeneous media; need a global identifier.
Solution: IP address — unique, universal, location-oriented (not name-oriented).

Classic IP Address Format

Two logical segments:
- Network ID (prefix)
- Host ID (suffix)
Originally classful (A/B/C) addressing.

Class A

1-bit leading $0$
Network bits: $8$
Hosts per network: $2^{24}-2 \approx 16\,777\,214$
Range: $1.0.0.0$ – $126.0.0.0$ (network) / $1.0.0.1$ – $126.255.255.254$ (host)

Class B

Leading bits $10$ ; $16$ -bit network field.
Networks: $2^{14}=16\,384$
Hosts per network: $65\,534$
Range: $128.0.0.0$ – $191.255.0.0$ (network)

Class C

Leading bits $110$ ; $24$ -bit network field.
Networks: $2^{21} \approx 2\,097\,152$
Hosts per network: $254$
Range: $192.0.0.0$ – $223.255.255.0$ (network)

Multihoming Concepts

Definition (precise): Entity (host or network) with several first-hop connections to a destination, possibly via multiple interfaces.
Alternative (ISP-centric): Having two or more connectivity providers for fault tolerance/traffic-engineering.
Each interface receives a unique IP address; addresses may differ in class or prefix.
Common multihomed devices:
- Laptops with Wi-Fi + Ethernet + 5G.
- Servers with multiple NICs.
- Border routers with links to multiple ISPs.

Multihoming Types

End-Host Multihoming (single machine, multiple interfaces/addresses)
End-Site Multihoming (whole network with prefixes from multiple providers)
- Diagram showed prefixes $1,..,n$ connected through multiple providers using Mobile Routers (MR).

Technical Issues

Strong End-System model: Outgoing packet must use source address belonging to the egress interface (strict binding).
Weak End-System model: Host may use any of its addresses irrespective of the interface (flexible but ambiguous).
Routing complication: Ensuring correct return path when a host owns multiple IPs.

Special IPv4 Addresses

Network Address: Host bits all $0$ (identifies network itself).
Direct Broadcast: Host bits all $1$ for a particular network.
Limited Broadcast: $255.255.255.255$ (local-network only).
“This Host” all-zero address used during boot (source only).
Loopback: $127.0.0.0/8$ (usually $127.0.0.1$ ).
Private Ranges (RFC 1918): $10/8$ , $172.16/12$ , $192.168/16$ .
Other blocks: Autoconfiguration $169.254/16$ , TEST-NET $192.0.2.0/24$ , $198.51.100.0/24$ , $203.0.113.0/24$ , Anycast relay $192.88.99.0/24$ , etc.

Subnet Masks & Subnetting (RFC 950)

Subnet Mask: 32-bit value; $1$ s mark network-prefix, $0$ s mark host-suffix.
- Default masks:
- Class A: $255.0.0.0$ (/8)
- Class B: $255.255.0.0$ (/16)
- Class C: $255.255.255.0$ (/24)
ANDing Rule: $\text{IP} \land \text{mask}$ performed on both source & destination; equality ⇒ same LAN.
Subnetting Motivation
- Scarcity of IPv4 addresses.
- Manageable routing tables.
- Single public NetID allocated, internally carved into sub-IDs.
Benefits: Address conservation, performance, scalability, enhanced security, technology mixing, classful limitation mitigation.
Extended Network Prefix adds Subnet Number field between network-ID & host-ID.
Design Questions (for any subnetting level)
1. Number of subnets needed now & future?
2. Host count per largest subnet now & future?
Special Subnet IDs: Historically all-0s & all-1s forbidden (router confusion), now allowed by modern gear.
Multiple Layers of Subnetting
- Hierarchical fixed-size approach (each layer uniform mask).
- Not true VLSM unless subnets of varying sizes coexist at the same layer.

Worked Example

Org holds 132.140.0.0/16 (Class B).
Could slice into eight /19 blocks:
1. 132.140.0.0/19
2. 132.140.32.0/19
3. 132.140.64.0/19
4. 132.140.96.0/19
5. 132.140.128.0/19
6. 132.140.160.0/19
7. 132.140.192.0/19
8. 132.140.224.0/19
Broadcast address for Subnet #3 (132.140.64.0/19) ⇒ 132.140.95.255.

Address Resolution & ICMP

Two-Level Addressing Rationale

Logical (IP) — universal scope.
Physical (MAC) — local scope.
Need mapping both ways for packet delivery.

ARP (Address Resolution Protocol)

Maps IP → MAC on local network using broadcast query/reply.
Caches mappings:
- Dynamic entries expire (default $10$ min, unused $2$ min).
- Static entries (e.g., broadcast FF:FF:FF:FF:FF:FF) persist.
Four Canonical Scenarios
1. Host → Host (same LAN).
2. Host → Remote host (ARP for default gateway MAC).
3. Router → Remote host (next-hop router).
4. Router → Host (same LAN).
Proxy ARP: Router answers ARP on behalf of other hosts, enabling subnetting illusion.
ARP Commands
- $arp\ -a$ — show cache
- $arp\ -d$ — delete entry
- $arp\ -s$ — add static entry
Gratuitous ARP: Node broadcasts its own IP/MAC to detect duplicates & update neighbor caches.
Reverse ARP (RARP): Legacy MAC → IP discovery (superseded by DHCP/BOOTP).

ARP Cache Operation

Each entry time-stamped on creation.
Removal timer: Unused $2$ min; actively used $10$ min.

Remote ARP Resolution Flow

Host classifies target IP as remote (mask check).
ARP cache checked for gateway MAC.
If missing, ARP broadcast for gateway’s MAC.
Frame sent to gateway; router repeats process toward next hop.

ICMP (Internet Control Message Protocol) — RFC 792

Network-layer protocol; unlike ARP, ICMP is encapsulated inside IP (type $1$ proto).
No error correction, only reporting.

Error Reporting Types

$3$ — Destination Unreachable
$4$ — Source Quench (deprecated congestion signal)
$11$ — Time Exceeded (TTL expired)
$12$ — Parameter Problem
$5$ — Redirect (better route advice)

Query / Management Types

$8$ / $0$ — Echo Request / Reply (\texttt{ping})
$13$ / $14$ — Timestamp Request / Reply
$17$ / $18$ — Address-Mask Request / Reply
$10$ / $9$ — Router Solicitation / Advertisement

Concluding Summary

Classful IP: Historical simplicity, but wasteful address use.
ARP family handles IP↔MAC mapping; includes RARP & Gratuitous ARP for special tasks.
ICMP delivers critical control/error messages enabling diagnostics (e.g., $ping$ , $traceroute$ ) and adaptive routing feedback.
Mastery of these foundational mechanisms is essential before advancing to upcoming topics: IP header specifics, routing protocols, transport-layer design, and security.