Day 7 - 8 IPv4 addressing

Layer 3 Overview

Layer 3 (Network Layer) provides connectivity between end hosts on DIFFERENT networks, not just within a local area network. Unlike Layer 2 which uses physical MAC addresses assigned at manufacture, Layer 3 uses logical IP addresses that are configured by network administrators. Routers operate at Layer 3 and are responsible for path selection, determining the best route for data to travel from source to destination across complex networks like the Internet.

IPv4 Address Structure

An IPv4 address is a 32-bit number divided into four 8-bit sections called octets, written in dotted decimal notation for human readability. Each octet can range from 0 to 255 (since 2^8 = 256 possible values). The address is split into two portions: the network portion (which identifies the network) and the host portion (which identifies specific devices on that network).

Binary Number System

Binary is base-2, meaning each digit position represents a power of 2, doubling as you move left. The 8-bit positions represent: 128, 64, 32, 16, 8, 4, 2, 1. To convert binary to decimal, add the values of positions containing a 1. To convert decimal to binary, subtract the largest possible value starting from 128, writing 1 if you can subtract, 0 if you cannot.

IPv4 Address Classes

IPv4 addresses are divided into five classes (A, B, C, D, E) based on the first octet. Classes A, B, and C are used for host addressing with different network/host bit distributions. Class D is reserved for multicast, and Class E is reserved for experimental purposes. The class determines the default prefix length and number of available networks versus hosts per network.

Special Addresses

The network address has all 0s in the host portion and identifies the network itself—it cannot be assigned to any device. The broadcast address has all 1s in the host portion and is used to send packets to all hosts on the network—it also cannot be assigned to a device. These two addresses reduce the usable host addresses in any network by 2.

Critical Numbers

• IPv4 address length: 32 bits (4 bytes)

• Octet size: 8 bits

• Octet range: 0-255

• Binary position values: 128-64-32-16-8-4-2-1

• Formula for usable hosts: 2^n - 2 (where n = host bits)

Broadcast Behavior

• Routers do NOT forward broadcast traffic between networks

• Broadcasts are limited to the local network segment

• Layer 3 broadcast IP uses all 1s in host portion

• Layer 2 broadcast MAC address is all Fs (FF:FF:FF:FF:FF:FF)

Router Interface Defaults

• Cisco router interfaces are administratively down by default

• Must use no shutdown command to enable interfaces

• Switch interfaces are UP by default (opposite of routers)

• Status column = Layer 1, Protocol column = Layer 2

Address Class Characteristics

• Class A: Fewer networks, more hosts per network

• Class C: More networks, fewer hosts per network

• Class B: Balanced between networks and hosts

• Loopback range (127.0.0.0/8) is reserved for testing

Dotted Decimal Notation: The human-readable format of IPv4 addresses using four decimal numbers (0-255) separated by dots (e.g., 192.168.1.1).

Octet: An 8-bit section of an IPv4 address; four octets make up the complete 32-bit address.

Prefix Length: The number of bits in the network portion of an IP address, written with a forward slash (e.g., /24 means 24 network bits).

Subnet Mask (Netmask): The dotted decimal representation of the prefix length, where network bits are all 1s and host bits are all 0s (e.g., 255.255.255.0 = /24).

Network Address: The first address in a network with all 0s in the host portion; identifies the network itself and cannot be assigned to hosts.

Broadcast Address: The last address in a network with all 1s in the host portion; used to send packets to all hosts on the network and cannot be assigned to hosts.

Loopback Address: Any address in the 127.0.0.0/8 range used to test the network stack of the local device without sending traffic externally.

BIA (Burned-In Address): Another term for MAC address, referring to the permanent hardware address assigned during manufacturing.

Administratively Down: Interface status indicating the interface has been manually disabled with the shutdown command.

IPv4 Address Classes Table

*Note: 0 and 127 are reserved in Class A

Subnet Masks to Memorize

• /8 = 255.0.0.0 (Class A default)

• /16 = 255.255.0.0 (Class B default)

• /24 = 255.255.255.0 (Class C default)

Binary Position Values (Right to Left)

1, 2, 4, 8, 16, 32, 64, 128

Special Address Ranges

• Loopback: 127.0.0.0 to 127.255.255.255

• Most common loopback: 127.0.0.1

1. Forgetting to subtract 2 from host calculations: Always remember network and broadcast addresses cannot be assigned to hosts. Formula is 2^n - 2, NOT 2^n.

2. Confusing prefix length with subnet mask: /24 and 255.255.255.0 are the same thing, just different notation. Cisco CLI uses subnet mask format.

3. Class A range confusion: Some sources say 0-127, others say 1-126. Remember that 0 and 127 are reserved, so usable Class A range is 1-126.

4. Router vs Switch defaults: Router interfaces are administratively DOWN by default; switch interfaces are UP by default.

5. Binary conversion errors: Practice the 128-64-32-16-8-4-2-1 method repeatedly. Most errors come from rushing through the calculation.

6. Network vs broadcast address identification: Network address has all 0s in host portion; broadcast has all 1s. Don't confuse them!

7. First and last usable addresses: First usable = network address + 1; Last usable = broadcast address - 1. Don't forget to add/subtract!

8. Status vs Protocol columns: Status = Layer 1 (physical); Protocol = Layer 2 (data link). You can't have Protocol UP if Status is DOWN.

"Can't Be Assigned" Mnemonic

"NBA" - Network and Broadcast Addresses cannot be assigned to hosts.

Binary Position Values

"1-2-4-8-16-32-64-128" - Practice this sequence forwards and backwards until automatic.

Class Ranges Trick

• Class A: 0-127 (starts with 0)

• Class B: 128-191 (starts with 10)

• Class C: 192-223 (starts with 110)

• Remember: Each class roughly doubles the starting number

Subnet Mask Quick Check

Count the 255s:

• One 255 = /8

• Two 255s = /16

• Three 255s = /24

Router Configuration Steps - "CIA-NE"

1. Config terminal (conf t)

2. Interface selection (int g0/0)

3. Address assignment (ip address X.X.X.X Y.Y.Y.Y)

4. No shutdown (no shut)

5. Exit and verify (show ip int brief)

Host Calculation Formula Memory

"Two to the N, minus TWO" - 2^n - 2 (n = number of host bits)

Example 1: Class C Network Analysis

Given: 192.168.1.0/24

• Network Address: 192.168.1.0

• First Usable: 192.168.1.1

• Last Usable: 192.168.1.254

• Broadcast: 192.168.1.255

• Total Hosts: 2^8 - 2 = 254

• Subnet Mask: 255.255.255.0

Example 2: Class B Network Analysis

Given: 172.16.0.0/16

• Network Address: 172.16.0.0

• First Usable: 172.16.0.1

• Last Usable: 172.16.255.254

• Broadcast: 172.16.255.255

• Total Hosts: 2^16 - 2 = 65,534

• Subnet Mask: 255.255.0.0

Example 3: Class A Network Analysis

Given: 10.0.0.0/8

• Network Address: 10.0.0.0

• First Usable: 10.0.0.1

• Last Usable: 10.255.255.254

• Broadcast: 10.255.255.255

• Total Hosts: 2^24 - 2 = 16,777,214

• Subnet Mask: 255.0.0.0

Example 4: Router Interface Configuration

Router(config)# interface gigabitethernet 0/0

Router(config-if)# ip address 192.168.1.254 255.255.255.0

Router(config-if)# description ## Connected to SW1 ##

Router(config-if)# no shutdown

Router(config-if)# exit

Router(config)# do show ip interface brief

32 bits or 4 bytes

An 8-bit section of an IPv4 address (IPv4 has 4 octets)

0 to 255 (2^8 = 256 possible values)

The human-readable format of IPv4 addresses using four decimal numbers separated by dots (e.g., 192.168.1.1)

Layer 3 (Network Layer)

Network portion and Host portion

Connectivity between DIFFERENT networks (not just within a LAN), logical addressing (IP), and path selection

No, broadcasts are limited to the local network and do not cross routers

1, 2, 4, 8, 16, 32, 64, 128

192 (128 + 64)

168 (128 + 32 + 8)

254 (128 + 64 + 32 + 16 + 8 + 4 + 2)

11111111

11000000

01111111

255 (when all bits are 1)

1-126 (technically 0-127, but 0 and 127 are reserved)

128-191

192-223

224-239 (reserved for multicast)

240-255 (reserved for experimental use)

0

10

110

/8

/16

/24

8 bits

24 bits

16 bits

24 bits

8 bits

16,777,214 (2^24 - 2)

65,534 (2^16 - 2)

254 (2^8 - 2)

255.0.0.0

255.255.0.0

255.255.255.0

/8

/16

/24

A dotted decimal representation of the prefix length where network bits are all 1s and host bits are all 0s

With a forward slash followed by the number of network bits (e.g., /24)

The first address in a network with all 0s in the host portion; identifies the network itself

The last address in a network with all 1s in the host portion; used to send packets to all hosts on the network

No, it cannot be assigned to any device

192.168.1.0

192.168.1.255

192.168.1.1 (network address + 1)

192.168.1.254 (broadcast address - 1)

127.0.0.0 to 127.255.255.255 (entire /8 network)

127.0.0.1

To test the network stack of the local device without sending traffic externally

The device responds to its own ping; traffic doesn't leave the device

2^n - 2, where n = number of host bits

To account for the network address and broadcast address, which cannot be assigned to hosts

Network address + 1

Broadcast address - 1

10.0.0.0

10.255.255.255

10.0.0.1

10.255.255.254

172.16.0.0

172.16.255.255

172.16.0.1

172.16.255.254

65,534 (2^16 - 2)

254 (2^8 - 2)

configure terminal (or conf t)

interface gigabitethernet 0/0 (or int g0/0)

ip address [ip-address] [subnet-mask]

no shutdown (or no shut)

show ip interface brief (or sh ip int br)

show interfaces [interface-name]

show interfaces description

description [text]

Administratively down (shutdown command applied by default)

Up (if connected) or Down (if not connected) - NOT administratively down

Use the "do" command before the show command (e.g., do show ip int brief)

Layer 1 (physical layer) status

Layer 2 (data link layer) status

The interface has been manually disabled with the shutdown command

No, Layer 2 cannot function if Layer 1 is down

Burned-In Address (the permanent MAC address)

en

int (minimum is "in", but "int" is commonly used)

Yes, "int gigabitethernet0/0" works the same as "int gigabitethernet 0/0"

How the IP address was assigned (manual, DHCP, etc.)

Class A; Network address: 43.0.0.0

43.255.255.255

16,777,214 (2^24 - 2)

Class B; Network address: 129.221.0.0

129.221.255.255

65,534 (2^16 - 2)

Class C; Network address: 209.211.3.0

209.211.3.255

254 (2^8 - 2)

Yes, they have the same network portion (192.168.1)

Because each interface connects to a different network, and the router needs an address in each network to route traffic

The router does not forward it; broadcasts are limited to the local network

FF:FF:FF:FF:FF:FF (all Fs)

All hosts on the 192.168.1.0/24 network (it's the broadcast address)

Fewer networks, but many hosts per network (24 host bits)

Many networks, but fewer hosts per network (8 host bits)

Multicast addresses (224.0.0.0 to 239.255.255.255)

Experimental purposes (240.0.0.0 to 255.255.255.255)

It's reserved for loopback addresses

The first 24 bits are the network portion (first 3 octets)

The first 16 bits are the network portion (first 2 octets)

The first 8 bits are the network portion (first octet)

Positions 128, 32, 8, and 2 (equals 170 in decimal)

00000000

11111111

255.0.0.0 (or the appropriate subnet mask)

no shutdown

Layer 1 is working, but there's a Layer 2 issue (possibly Ethernet framing or encapsulation problem)

Yes, you can type "interface [name]" without exiting to global config first

To document what each interface connects to or its purpose (optional but helpful)

Before the Exam:

1. Master binary conversion - Practice until you can convert without writing it down

2. Memorize the class ranges - You should instantly recognize 10.x.x.x as Class A

3. Know the formulas - 2^n - 2 for usable hosts should be automatic

4. Practice subnet mask conversion - /24 = 255.255.255.0 should be instant recall

5. Understand the concepts - Don't just memorize; understand WHY routers stop broadcasts

During the Exam:

1. Write down binary values - Start each question by writing 128-64-32-16-8-4-2-1 on scratch paper

2. Watch for trick questions - They may ask for "total addresses" vs "usable addresses"

3. Double-check network vs broadcast - All 0s = network, all 1s = broadcast

4. Verify your class identification - Check the first octet range carefully

5. Use process of elimination - If unsure, eliminate obviously wrong answers first

Common Question Types:

• Binary to decimal conversion (and vice versa)

• Identifying address class from first octet

• Calculating usable hosts

• Finding network, broadcast, first/last usable addresses

• Matching prefix lengths to subnet masks

• Router configuration commands

• Interpreting "show" command outputs