Review from Module 3

Review: From Layer 2 to Layer 3  

• Module 3 covered switches and MAC addresses (Layer 2).
• Today’s question: How do we communicate beyond our local network?
• MAC addresses only work locally—switches don’t forward to other networks.
• We need IP addresses (Layer 3) to route traffic across networks.

The Key Difference

MAC = local delivery (within your network) IP = end-to-end delivery (across the internet)

Learning Outcomes  

After completing this module, you will be able to:

• Explain the role of Layer 3 (IP) addressing and how it differs from Layer 2 (MAC) addressing.
• Describe ARP (Address Resolution Protocol) and how it maps IP addresses to MAC addresses.
• Interpret IPv4 addresses in dotted decimal notation and understand the role of subnet masks.
• Calculate network addresses, broadcast addresses, and usable host ranges using CIDR notation.
• Apply VLSM (Variable Length Subnet Masking) to create subnets of different sizes.
• Identify private IP ranges (RFC 1918) and explain NAT (Network Address Translation).
• Configure IP addresses, subnet masks, and default gateways on network devices.
• Use troubleshooting tools including ping, arp, ipconfig/ifconfig, and traceroute.
• Compare IPv4 and IPv6 addressing formats and describe IPv6 adoption strategies.

1 IP Fundamentals and Address Resolution

1.1 IPv4 Header and Address Roles

This section introduces IPv4 packet structure and the relationship between Layer 2 and Layer 3 addressing during local delivery and routing.

The IPv4 Datagram Header  

• IP wraps data in a datagram (like frames at Layer 2).
• Header contains addressing and control information.

• Key fields:

  • Version: 4 for IPv4
  • TTL: Time to Live (hop limit)
  • Source IP: Sender’s address
  • Destination IP: Receiver’s address
  • Protocol: What’s inside (TCP, UDP)

TTL Prevents Loops

Each router decrements TTL by 1. When TTL reaches 0, the packet is discarded—prevents infinite loops!

Layer 2 vs Layer 3 Addressing  

MAC Address (Layer 2)

• Hardware address burned into NIC
• Works only on local network
• Changes at each router hop
• Example: 00:1A:2B:3C:4D:5E

IP Address (Layer 3)

• Logical address assigned by admin
• Works across networks
• Stays the same end-to-end
• Example: 192.168.1.100

Mailing Analogy

IP address = Full mailing address (123 Main St, Springfield, USA)

MAC address = Apartment number (Apt 4B—only meaningful inside the building)

How Routers Use Both Addresses  

Key Insight

IP addresses stay the same from source to destination. MAC addresses change at every hop—the router creates a new frame for each network segment.

Address Resolution Protocol (ARP)  

• Problem: You know the destination IP, but need the MAC to send a frame.
• Solution: ARP asks “Who has this IP?”
• ARP Request: Broadcast to all devices—“Who has 192.168.1.50?”
• ARP Reply: Target responds with its MAC address (unicast).
• Result is cached in the ARP table.

Security Note

ARP has no authentication. ARP spoofing attacks send fake replies to redirect traffic!

ARP Table and Cache  

• ARP results are stored in the ARP cache (also called ARP table).
• Avoids broadcasting for every single packet.
• Entries expire after a timeout (typically 2–20 minutes).
• View with: arp -a (Windows/Linux/Mac)

Common ARP Commands

arp -a — Display all entries arp -d [IP] — Delete an entry arp -s [IP] [MAC] — Add static entry

Sample ARP Table Output

Troubleshooting Tip

Empty ARP table? No local communication is happening—check Layer 1 and 2 first!

Unicast, Broadcast, Multicast, and Anycast  

Common Uses

Unicast: Most traffic (web, email) Broadcast: ARP, DHCP discovery

Common Uses

Multicast: Video streaming, routing updates Anycast: DNS, CDNs (find closest server)

Case Study: Homer’s Home Network Mystery  

Case Study: Homer’s Home Network Mystery
Homer Simpson just set up a new PC in his home office. He runs some tests:

• He can ping 127.0.0.1 (loopback) — Success!
• He can ping his own IP 192.168.1.50 — Success!
• He cannot ping Marge’s laptop 192.168.1.51 — Fails!
• He cannot reach the internet — Fails!

Homer runs arp -a and sees an empty ARP table.

Review Questions

1.

What does an empty ARP table suggest about network communication?

2.

What Layer 1 or Layer 2 issue might cause this?

3.

What should Homer check first?

Case Study Solution: Homer’s Home Network Mystery  

Solution: Homer’s Home Network Mystery

1.

Empty ARP table means no successful local communication—ARP requests aren’t being answered (or sent).

2.

Likely Layer 1/2 causes:

• Ethernet cable unplugged or damaged
• NIC disabled in operating system
• Switch port is dead or disabled
• Wrong VLAN assignment on switch port

3.

Homer should check (in order):

• Cable connection at PC and switch
• Link lights on NIC and switch port
• NIC status in Windows (Network Connections)

Key Lesson

If the ARP table is empty, the problem is almost always Layer 1 or Layer 2—not the IP configuration. Check physical connectivity first!

2 Subnetting and Host Addressing

2.1 Address Structure and Masks

This section covers IPv4 addressing structure, subnet masks, and host range calculations used in practical network design.

IPv4 Address Format  

• IPv4 addresses are 32 bits long.
• Written as four octets in decimal, separated by dots.
• Each octet ranges from 0 to 255.
• Total possible addresses: about 4.3 billion.
• Not enough for today’s internet!

Why 0–255?

Each octet is 8 bits. 2⁸ = 256 possible values (0 through 255).

Network Masks Explained  

• Every IP address has two parts:

  • Network portion: Which network?
  • Host portion: Which device?
• The subnet mask defines where to split.
• Mask uses 1s for network bits, 0s for host bits.

Analogy

Think of a phone number: area code (network) + local number (host).

Subnet Mask Examples  

Why “Usable” Is Less

Two addresses are always reserved:

• Network address – all 0s in host portion
• Broadcast address – all 1s in host portion

Formula

Usable hosts = 2ⁿ −2

where n = number of host bits

Example: /24 has 8 host bits 2⁸ −2 = 256 −2 = 254 hosts

Calculating Host Address Ranges  

Given Information

• Network: 192.168.1.0/24
• Subnet Mask: 255.255.255.0

Step-by-Step Calculation

1. Network Address: 192.168.1.0 – host bits all 0

2. First Usable Host: 192.168.1.1 – network + 1

3. Last Usable Host: 192.168.1.254 – broadcast - 1

4. Broadcast Address: 192.168.1.255 – host bits all 1

Default Gateway  

• The default gateway is the router that connects your network to other networks.
• If destination IP is NOT on your local subnet → send to the gateway.
• Gateway must be on the same subnet as the host.
• Typically the first or last usable address (e.g., .1 or .254).

Common Mistake

If the gateway is on a different subnet than the host, the host cannot reach it—and cannot reach anything outside the local network!

PC Configuration

IP: 192.168.1.100 Mask: 255.255.255.0 Gateway: 192.168.1.1

Case Study: Moe’s Tavern Network Problem  

Case Study: Moe’s Tavern Network Problem
Moe is setting up a new point-of-sale (POS) system at the tavern to track Duff Beer sales. The system is configured as follows:

Barney’s laptop (192.168.10.25) can ping the POS system just fine. However, the POS system cannot reach the internet or the beer distributor’s ordering website.

Review Questions

1.

What’s wrong with this configuration?

2.

Why can Barney reach the POS but the POS can’t reach the internet?

3.

What should the gateway address be?

Case Study Solution: Moe’s Tavern Network Problem  

Solution: Moe’s Tavern Network Problem

1.

The gateway 192.168.20.1 is on a different subnet than the POS system (192.168.10.x).

2.

Why Barney can reach POS but POS can’t reach internet:

• Barney and POS are on the same subnet—local traffic doesn’t need the gateway
• Internet traffic requires the gateway, which is unreachable (different subnet)

3.

Gateway should be on the same subnet, such as 192.168.10.1.

Key Lesson

The default gateway must be on the same subnet as the host. This is one of the most common misconfigurations!

3 Address Planning Strategies

3.1 Classful, CIDR, and VLSM

Address planning evolved from classful limits to CIDR and VLSM for efficient allocation and scalable route summarization.

Classful Addressing (Legacy)  

• The original IP design divided addresses into classes based on the first octet.
• Each class had a fixed network/host boundary—no flexibility!

The Problem

Very wasteful! A company needing 300 hosts had to get a Class B (65,534 addresses).

The Solution

CIDR (Classless Inter-Domain Routing) replaced classes with flexible prefix lengths.

Public vs Private Addressing  

Public IP Addresses

• Routable on the internet
• Must be globally unique
• Assigned by ISPs (from IANA/RIRs)
• Limited supply (IPv4 exhaustion!)

Private IP Addresses

• For internal networks only
• NOT routable on internet
• Can be reused by any organization
• Free to use—no registration needed

Private Address Ranges

NAT Makes It Work

NAT (Network Address Translation) converts private addresses to public at the router—this is how your home network reaches the internet!

Other Reserved Address Ranges  

Loopback (127.0.0.1)

• Always refers to “yourself”
• Tests if TCP/IP is working
• Never leaves the device
• Also called localhost

APIPA (169.254.x.x)

If you see this address, the device couldn’t get a DHCP address!

Troubleshoot:

• DHCP server down?
• Network cable unplugged?
• Wrong VLAN?

CIDR: Classless Inter-Domain Routing  

• CIDR replaced classful addressing in 1993.
• Much more flexible—any prefix length allowed!
• Notation: IP address + slash + prefix length.
• Prefix = number of network bits.

Reading CIDR Notation

192.168.1.0/24

• Network: 192.168.1.0
• Prefix: 24 bits for network
• Remaining: 8 bits for hosts

Quick Math

Addresses = 2^{(32−prefix)}

Example: /26 2^(32−26) = 2⁶ = 64 addresses

Variable Length Subnet Masks (VLSM)  

• VLSM allows different subnet sizes within the same network.
• Assign subnets based on actual need—no wasted addresses!
• Always allocate largest subnets first.

Example Requirement

Company needs: → Sales: 100 hosts → Engineering: 50 hosts → Management: 25 hosts → Point-to-point link: 2 hosts

Result

Used 220 addresses for 177 hosts. Without VLSM: would need 512+ addresses!

Case Study: Springfield Elementary Network Design  

Case Study: Springfield Elementary Network Design
Principal Skinner needs to design the school network with three segments. The district IT department has assigned him 172.16.50.0/24 (256 addresses).

Skinner wants to use VLSM to efficiently allocate addresses without waste.

Review Questions

1.

What is the smallest subnet size that can fit 28 hosts?

2.

List appropriate subnet assignments for each segment.

3.

How many addresses will be “wasted” (unused or reserved)?

Case Study Solution: Springfield Elementary Network Design  

Solution: Springfield Elementary Network Design

1.

28 hosts needs at least 30 usable addresses → /27 (32 addresses, 30 usable).

2.

Subnet assignments (largest first):

Address Accounting

Used: 32 + 16 + 8 = 56 addresses For: 28 + 12 + 6 = 46 devices Reserved (net/bcast): 6 addresses “Wasted”: 4 addresses (headroom)

Key Lesson

VLSM lets you right-size subnets. Always assign largest first, then work down. Leave room for growth!

4 IP Configuration and Diagnostics

4.1 Host Tools and Connectivity Testing

This section reviews host-side IP tools and a systematic approach to connectivity testing and troubleshooting.

ipconfig (Windows)  

Common Commands

ipconfig Shows IP, mask, gateway

ipconfig /all Adds MAC, DHCP server, DNS

ipconfig /release Release current DHCP lease

ipconfig /renew Request new DHCP lease

ipconfig /flushdns Clear the DNS cache

Sample Output: ipconfig /all

Ethernet adapter Local Area Connection:

Troubleshooting Tip

See 169.254.x.x? DHCP failed! See 0.0.0.0? No IP assigned at all.

ifconfig and ip (Linux)  

Legacy: ifconfig

ifconfig Show all interface configuration

ifconfig eth0 Show specific interface

ifconfig eth0 down Disable an interface

ifconfig eth0 up Enable an interface

Note

ifconfig is deprecated on many modern Linux distributions. Use ip instead!

Modern: ip command

ip addr (or ip a) Show IP addresses

ip link Show interface status (up/down)

ip route Show routing table and gateway

ip neigh Show ARP cache (neighbors)

Sample: ip addr

2: eth0: <UP,BROADCAST> inet 192.168.1.50/24 link/ether 00:1a:2b:3c:4d:5e

The arp Command  

Common Commands

arp -a Display entire ARP cache

arp -a 192.168.1.1 Display entry for specific IP

arp -d 192.168.1.1 Delete an entry from cache

arp -s 192.168.1.1 00-1a-2b-3c-4d-5e Add a static entry manually

When to Use

• Verify local connectivity
• Troubleshoot duplicate IPs
• Check for ARP poisoning
• Force re-resolution of MAC

Sample Output: arp -a

Interface: 192.168.1.100

Dynamic vs Static

Dynamic: Learned via ARP, expires after timeout

Static: Manually configured, never expires

ping: The Essential Connectivity Test  

• ping tests reachability using ICMP echo request/reply.
• Measures round-trip time (RTT) in milliseconds.
• TTL in response indicates hops traveled.
• Works on Windows, Linux, Mac.

Common Options

ping 192.168.1.1 Basic connectivity test

ping -t 192.168.1.1 Continuous ping – Windows

ping -c 5 192.168.1.1 Send 5 pings – Linux/Mac

Sample Output

Pinging 192.168.1.1 with 32 bytes of data:

Reply from 192.168.1.1: bytes=32 time=2ms TTL=64 Reply from 192.168.1.1: bytes=32 time=1ms TTL=64 Reply from 192.168.1.1: bytes=32 time=1ms TTL=64 Reply from 192.168.1.1: bytes=32 time=2ms TTL=64

Common Results

Reply – Success! Host is reachable

Request timed out – No response received

Destination unreachable – Routing problem

Ping Troubleshooting Methodology  

If Step 1 Fails

TCP/IP stack is broken. Reinstall network drivers or reset TCP/IP stack.

If Step 2 Fails

NIC is not configured properly. Check IP settings, cable, NIC driver.

If Step 3 Fails

Local network issue. Check cable, switch port, gateway address, ARP table.

If Step 4 Fails

Routing or remote issue. Check gateway config, ISP connection, firewall.

5 IPv6 Fundamentals

5.1 Addressing, Types, and Transition

The final section introduces IPv6 structure, address types, and migration approaches from IPv4 environments.

Why IPv6? The Problem with IPv4  

• IPv4 has only about 4.3 billion addresses.
• Already exhausted! IANA ran out in 2011.
• Workarounds like NAT and CIDR helped, but add complexity.
• IPv6 provides a permanent solution.

IPv6 Address Space

2¹²⁸ = 340 undecillion addresses

That’s 340,282,366,920,938,463,463,374,607,431,768,211,456 addresses!

IPv6 Benefits

• Virtually unlimited addresses
• Simpler header format
• Built-in IPsec support
• No need for NAT
• Auto-configuration – SLAAC
• Better multicast support

Perspective

IPv6 could assign a unique address to every atom on Earth’s surface... and still have addresses left over!

IPv6 Address Format  

• 128 bits long – 4x longer than IPv4.
• Written as 8 groups of 4 hex digits.
• Groups separated by colons.
• Letters can be uppercase or lowercase.

Full Format

2001:0db8:85a3:0000:0000:8a2e:0370:7334

Simplification Rules

Rule 1: Drop leading zeros in each group

2001:0db8:85a3:0000:0000:8a2e:0370:7334

↓

2001:db8:85a3:0:0:8a2e:370:7334

Rule 2: Replace ONE set of consecutive zero groups with ::

2001:db8:85a3:0:0:8a2e:370:7334

↓

2001:db8:85a3::8a2e:370:7334

Important: Use :: Only Once!

You can only use :: once per address. Using it twice would make the address ambiguous – you wouldn’t know how many zero groups each :: represents.

IPv6 Network Prefixes and Address Types  

• IPv6 uses prefix length instead of subnet mask.
• Written with slash notation: /64
• Standard subnet prefix: /64
• First 64 bits = network, last 64 bits = interface ID.

Common IPv6 Address Types

Example Address

2001:db8:1234:5678::1/64

Prefix: 2001:db8:1234:5678 Interface ID: ::1

IPv6 Address Types Explained  

Global Unicast – GUA

• Public, routable addresses
• Equivalent to IPv4 public IPs
• Start with 2 or 3
• Assigned by ISPs

Link-Local – fe80::

• Auto-configured on every interface
• Only valid on local segment
• NOT routable
• Used for neighbor discovery

Always Present

Every IPv6 interface has a link-local address, even without DHCP or manual config!

Unique Local – fc00::/7

• Private addresses
• Like IPv4 10.x.x.x or 192.168.x.x
• NOT routable on internet
• For internal use only

Multicast – ff00::/8

• One-to-many communication
• Replaces IPv4 broadcast
• ff02::1 = all nodes
• ff02::2 = all routers

Loopback

::1 is the IPv6 loopback – equivalent to IPv4’s 127.0.0.1

IPv4 to IPv6 Transition  

Dual Stack

• Run IPv4 AND IPv6 simultaneously
• Most common transition method
• Devices have both addresses
• Gradual migration over time

Tunneling

• Encapsulate IPv6 inside IPv4
• Cross IPv4-only networks
• Types: 6to4, Teredo, ISATAP
• Adds overhead

Translation – NAT64

• Convert between protocols
• IPv6-only hosts reach IPv4 servers
• More complex to manage
• Last resort option

Recommendation

Dual stack is the preferred transition method. Run both protocols until IPv4 can be safely retired. Most modern operating systems support dual stack by default.

Module Summary

Module 4.0 Summary  

Key Concepts:

• IP (Layer 3) provides end-to-end addressing; MAC (Layer 2) provides local delivery. ARP resolves IP to MAC.
• IPv4 addresses: 32-bit dotted decimal (e.g., 192.168.1.100). Subnet mask divides network and host portions.
• CIDR notation (e.g., /24) replaced classful addressing. Usable hosts = 2ⁿ − 2 (subtract network and broadcast addresses).
• VLSM (Variable Length Subnet Masking) allows subnets of different sizes for efficient IP allocation.
• Private IP ranges: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16 (RFC 1918). Use NAT for internet access.
• Address types: Unicast (one-to-one), Broadcast (one-to-all on subnet), Multicast (one-to-group), Anycast (one-to-nearest).
• Default gateway must be on the same subnet as the host. TTL (Time to Live) prevents routing loops.

Troubleshooting & IPv6:

• Tools: ipconfig/ifconfig (view config), ping (test connectivity), arp (view MAC mappings), traceroute (show path).
• Methodology: Ping loopback (127.0.0.1) → local gateway → remote host to isolate problems.
• 169.254.x.x = APIPA (DHCP failure). Empty ARP table indicates Layer 1/2 problem.
• IPv6: 128-bit addresses in hex notation (e.g., 2001:db8::1). Use :: to compress zeros.
• IPv6 types: Link-local (fe80::/10, auto-configured), Global unicast (2000::/3), Unique local (fc00::/7).
• Dual stack: Run IPv4 and IPv6 simultaneously during transition period.

Conclusion

This module covered IPv4 and IPv6 addressing, subnetting with CIDR and VLSM, address resolution with ARP, and troubleshooting tools. You learned to calculate subnet masks, plan IP address schemes, and diagnose connectivity problems systematically. In the next module, we’ll explore routing protocols and how packets travel between networks.