Chapter 3

🏠 IP Addresses Made Simple

By Sys-Metrics· · 60 min chapter

🎯 Your Gateway to Understanding Networks

IP addresses are the foundation of all networking. Think of them as the postal system of the internet - without proper addresses, nothing gets delivered to the right place. By the end of this chapter, you'll understand IP addressing better than most IT professionals!

🎯 Chapter Goals: Master IPv4 structure, understand subnetting fundamentals, calculate networks like a pro, and configure IP addresses on Cisco devices!

🏠 What is an IP Address? (Deep Understanding)

An IP address is like a complete mailing address for devices on a network. Just like your home address has multiple parts that get more specific, IP addresses have structure too.

Real-World Address Analogy

Postal Address:
John Smith
123 Main Street ← Specific house (HOST)
Springfield ← City/Area (NETWORK)
Illinois ← State
USA ← Country
IP Address:
192.168.1.100
└─ 100 ← Specific device (HOST)
└─ 192.168.1 ← Network area (NETWORK)

IPv4 Structure Breakdown

IPv4 addresses are 32-bit numbers divided into 4 octets (8-bit sections):

192.168.1.100
192 . 168 . 1 . 100
• Each octet ranges from 0-255 (8 bits each)
• Total address space: 32 bits
• Binary: 11000000.10101000.00000001.01100100
🧠 Memory Trick: Each octet can be 0-255 because that's what fits in 8 bits (2^8 = 256 possible values, starting from 0).

🎭 The Two Faces of Every IP Address

Every IP address has two parts, like a coin with two sides. Understanding this concept is crucial for networking.

Network Portion - The Neighborhood

Purpose

Identifies which network/subnet the device belongs to

Analogy

Like the "Springfield, Illinois" part of an address - it tells routers which neighborhood to deliver to

Function

Used by routers to make forwarding decisions between networks

Host Portion - The Specific House

Purpose

Identifies the specific device within that network

Analogy

Like "123 Main Street" - the exact house within the neighborhood

Function

Used by switches to deliver frames to the correct device within the LAN

Subnet Mask - The Dividing Line

The subnet mask tells us where to split the IP address between network and host portions:

IP Address: 192.168.1.100
Subnet Mask: 255.255.255.0
192 . 168 . 1 . 100
Network Address: 192.168.1.0
Host Address: 100 (within the network)
Green: Network portion | Yellow: Host portion

🔢 Binary Made Simple (No Fear Required!)

Binary is just counting with only 2 digits (0 and 1) instead of 10. You don't need to be a math genius - just understand the pattern.

Binary Position Values

Position 8 7 6 5 4 3 2 1
Value 128 64 32 16 8 4 2 1
Binary (192) 1 1 0 0 0 0 0 0
Calculation: (1×128) + (1×64) = 192
Green: Bit is ON (1) | Red: Bit is OFF (0)

Quick Binary Conversion Tricks

Common Network Values

128=10000000, 192=11000000, 224=11100000, 240=11110000, 248=11111000, 252=11111100, 254=11111110, 255=11111111

Pattern Recognition

Subnet masks always have consecutive 1s followed by consecutive 0s. Example: 11111111.11111111.11111111.00000000 = 255.255.255.0

🎯 Pro Tip: You don't need to memorize all binary. Just understand that 1s mark the network portion and 0s mark the host portion in subnet masks.

🍕 Subnetting with Pizza Slices

Subnetting is like cutting a pizza into smaller slices. You start with one big network and divide it into smaller networks.

The Pizza Analogy

🍕 Original Pizza (Network)

  • Whole pizza: 192.168.1.0/24
  • Can feed: 254 people (hosts)
  • Problem: Everyone in one room (broadcast domain)
  • Solution: Cut into smaller slices!

🍕 Pizza Slices (Subnets)

  • Slice 1: 192.168.1.0/25 (126 hosts)
  • Slice 2: 192.168.1.128/25 (126 hosts)
  • Benefit: Smaller broadcast domains
  • Better: More efficient, better security

CIDR Notation Explained

CIDR (Classless Inter-Domain Routing) notation uses "/" followed by a number:

/24

24 bits for network, 8 bits for host. Subnet mask: 255.255.255.0

/25

25 bits for network, 7 bits for host. Subnet mask: 255.255.255.128

/26

26 bits for network, 6 bits for host. Subnet mask: 255.255.255.192

/27

27 bits for network, 5 bits for host. Subnet mask: 255.255.255.224

Host Calculation Formula

Number of hosts per subnet = 2^(host bits) - 2

/24 network: 2^8 - 2 = 254 hosts
/25 network: 2^7 - 2 = 126 hosts
/26 network: 2^6 - 2 = 62 hosts
/27 network: 2^5 - 2 = 30 hosts
/28 network: 2^4 - 2 = 14 hosts
/29 network: 2^3 - 2 = 6 hosts
/30 network: 2^2 - 2 = 2 hosts (point-to-point)
🤔 Why subtract 2? Every subnet loses 2 addresses: the network address (all host bits = 0) and broadcast address (all host bits = 1).

🌍 Private vs Public IP Addresses

🏠 Private IP Address Ranges

These addresses are like your home's internal room numbers - they work inside your house but mean nothing to the outside world.

Class A Private: 10.0.0.0/8

Range: 10.0.0.0 - 10.255.255.255
Hosts: 16,777,214 per network
Use: Large enterprises, ISPs

Class B Private: 172.16.0.0/12

Range: 172.16.0.0 - 172.31.255.255
Hosts: 65,534 per network
Use: Medium businesses

Class C Private: 192.168.0.0/16

Range: 192.168.0.0 - 192.168.255.255
Hosts: 254 per /24 network
Use: Home networks, small offices

🌍 Public IP Addresses

Definition

Globally unique addresses routable on the internet

Assignment

Managed by IANA, allocated to ISPs, then to customers

Cost

Limited supply, especially IPv4 - companies pay for blocks

Examples

8.8.8.8 (Google DNS), 1.1.1.1 (Cloudflare), 208.67.222.222 (OpenDNS)

Special Purpose Addresses

127.0.0.1 (Loopback)

Always refers to "this device" - used for testing

169.254.x.x (APIPA)

Automatic Private IP Addressing - self-assigned when DHCP fails

224.0.0.0-239.255.255.255

Multicast addresses - one-to-many communication

0.0.0.0

Default route or "unknown" address

255.255.255.255

Broadcast to all devices on local network

🧮 Practical Subnetting Examples

Example 1: Basic Subnetting

Scenario: You have 192.168.10.0/24 and need 4 subnets with about 50 hosts each.

Solution: Use /26 (4 subnets, 62 hosts each)
Subnet 1
192.168.10.0/26
Range: .0 - .63
Subnet 2
192.168.10.64/26
Range: .64 - .127
Subnet 3
192.168.10.128/26
Range: .128 - .191
Subnet 4
192.168.10.192/26
Range: .192 - .255

Example 2: Variable Length Subnet Masking (VLSM)

Scenario: You need different sized subnets from 192.168.20.0/24:

  • Sales department: 100 hosts
  • Engineering: 50 hosts
  • IT: 20 hosts
  • Management: 10 hosts
Solution (largest to smallest):
Sales: 192.168.20.0/25 (126 hosts) ✓
Engineering: 192.168.20.128/26 (62 hosts) ✓
IT: 192.168.20.192/27 (30 hosts) ✓
Management: 192.168.20.224/28 (14 hosts) ✓

Subnetting Quick Reference

/24 = 255.255.255.0 = 1 subnet, 254 hosts
/25 = 255.255.255.128 = 2 subnets, 126 hosts each
/26 = 255.255.255.192 = 4 subnets, 62 hosts each
/27 = 255.255.255.224 = 8 subnets, 30 hosts each
/28 = 255.255.255.240 = 16 subnets, 14 hosts each
/29 = 255.255.255.248 = 32 subnets, 6 hosts each
/30 = 255.255.255.252 = 64 subnets, 2 hosts each

⚙️ Configuring IP Addresses on Cisco Devices

Configuring Switch Management IP

Switches need IP addresses for management (SSH, web interface, SNMP):

Switch> enable
Switch# configure terminal
Switch(config)# interface vlan 1
Switch(config-if)# ip address 192.168.1.10 255.255.255.0
Switch(config-if)# no shutdown
Switch(config-if)# exit
Switch(config)# ip default-gateway 192.168.1.1
Switch(config)# end
Switch# copy running-config startup-config

Configuring Router Interface IP

Router interfaces need IP addresses to route between networks:

Router> enable
Router# configure terminal
Router(config)# interface gigabitethernet 0/0
Router(config-if)# ip address 192.168.1.1 255.255.255.0
Router(config-if)# no shutdown
Router(config-if)# description LAN Interface
Router(config-if)# exit
Router(config)# end
Router# copy running-config startup-config

Useful Verification Commands

show ip interface brief

Quick status of all interfaces and their IPs

show interface vlan 1

Detailed info about switch management interface

show ip route

Display routing table (routers only)

ping [ip-address]

Test connectivity to another device

show arp

Display ARP table (IP to MAC mappings)

🛠️ Hands-On Labs

Lab 1: IP Address Analysis

  1. Given these IP addresses, identify network and host portions:
    • 192.168.50.25/24
    • 10.1.1.100/16
    • 172.16.10.50/20
  2. Calculate number of hosts for each:
    • What's the broadcast address for each?
    • What's the first and last usable host address?

Lab 2: Subnetting Practice

  1. Subnet 192.168.100.0/24 into 8 equal subnets
  2. List the network address for each subnet
  3. How many hosts per subnet?
  4. What subnet mask will you use?

Lab 3: Cisco Device Configuration

  1. Configure a switch management IP:
    • IP: 192.168.1.20/24
    • Default gateway: 192.168.1.1
    • Test with ping to gateway
  2. Configure router interface:
    • GigE0/0: 10.1.1.1/24
    • GigE0/1: 10.2.2.1/24
    • Verify with show ip interface brief

Lab 4: VLSM Challenge

Design subnets for a company using 172.16.0.0/16:

  • Head Office: 500 users
  • Branch 1: 200 users
  • Branch 2: 100 users
  • Branch 3: 50 users
  • Point-to-point links: 6 links needed
🎯 Challenge Goal: Design this network efficiently with minimal waste of IP addresses. Use VLSM principles!

🚨 Common IP Addressing Mistakes

Critical Errors to Avoid

Host Address as Network Address

❌ Using 192.168.1.0 as a host IP in /24 network
✅ Use 192.168.1.1-192.168.1.254 for hosts

Broadcast Address as Host

❌ Using 192.168.1.255 as host IP in /24 network
✅ Reserve .255 as broadcast address

Wrong Subnet Mask

❌ Using 255.255.255.0 when you meant /25
✅ Verify subnet mask matches your design

Overlapping Subnets

❌ Creating subnets that overlap address ranges
✅ Plan subnets sequentially without gaps

Not Saving Configuration

❌ Configuring IP addresses without copy run start
✅ Always save your IP configurations

Troubleshooting IP Issues

Cannot Ping Gateway

Check: IP address, subnet mask, physical connectivity, interface status

Wrong Network Communication

Check: Subnet mask calculation, VLAN configuration, routing table

IP Address Conflicts

Check: DHCP scope, static IP assignments, ARP table

📖 Chapter Summary

  • IP Structure: 32-bit addresses with network and host portions
  • Subnet Masks: Define boundary between network and host bits
  • CIDR Notation: /24, /25, /26 format for subnet masks
  • Private vs Public: 10.x.x.x, 172.16-31.x.x, 192.168.x.x vs internet-routable
  • Subnetting: Dividing networks into smaller, manageable pieces
  • VLSM: Variable length subnetting for efficient IP usage
  • Cisco Configuration: Interface IP setup, verification commands
  • Binary Basics: Understanding the math behind IP addressing
🎯 Mastery Achieved! You now understand IP addressing better than most IT professionals. These skills form the foundation for all advanced networking concepts.

📝 IP Addressing Mastery Quiz

1. How many host addresses are available in a /26 network? 62 hosts (2^6 - 2 = 64 - 2 = 62)

2. What's the broadcast address for 192.168.10.50/25? 192.168.10.127 (host is in first half of /24, so broadcast is .127)

3. Which private IP range has the most addresses? 10.0.0.0/8 (16.7 million addresses)

4. How do you save IP configuration on Cisco device? copy running-config startup-config

5. What command shows all interface IPs quickly? show ip interface brief

6. In 172.16.50.25/20, what's the network address? 172.16.48.0 (20 bits = 255.255.240.0, so network increments by 16)

7. What's wrong with configuring 192.168.1.0 as a host IP? It's the network address - first and last addresses are reserved

8. How many /26 subnets can you create from a /24 network? 4 subnets (borrowing 2 host bits: 2^2 = 4)

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