The IPv4 Protocol

The IPv4 Protocol

1. IPv4 Overview

IPv4 (Internet Protocol version 4) is the core protocol of the Internet. It provides the network layer with a connectionless datagram service.

Basic Characteristics

• Connectionless protocol: every packet is handled independently
• Best effort service: reliable transfer is not guaranteed
• 32-bit addresses: roughly 4.3 billion addresses
• Globally unique: every device has a unique IP address

2. The IPv4 Datagram Structure

Version and Header Length

• Version (4 bits): the protocol version, 4 for IPv4
• Header length (4 bits): measured in 4-byte units, at most 60 bytes

Type of Service

• Precedence (3 bits): the packet priority
• D bit: the delay requirement
• T bit: the throughput requirement
• R bit: the reliability requirement

Total Length and Identification

• Total length (16 bits): the length of the whole packet
• Identification (16 bits): used to reassemble fragments

Flags and Fragment Offset

• Flags (3 bits):
  • DF bit: the do-not-fragment flag
  • MF bit: the more-fragments flag
• Fragment offset (13 bits): the position of the fragment within the original packet

Time To Live and Protocol

• Time To Live (8 bits): the maximum number of hops the packet may take
• Protocol (8 bits): the upper layer protocol type (TCP=6, UDP=17)

Checksum and Addresses

• Header checksum (16 bits): an integrity check of the header
• Source address (32 bits): the sender’s IP address
• Destination address (32 bits): the receiver’s IP address

Options and Data

• Options (variable): optional fields
• Data (variable): upper layer protocol data

3. IPv4 Addresses

Address Structure

• 32 bits in binary: divided into four 8-bit segments
• Dotted decimal: each segment written in decimal and separated by dots
• Network part: identifies the network
• Host part: identifies the host

Address Classes

Class A addresses:

• Range: 1.0.0.0 - 126.255.255.255
• Network bits: 8 bits
• Host bits: 24 bits
• Default mask: 255.0.0.0

Class B addresses:

• Range: 128.0.0.0 - 191.255.255.255
• Network bits: 16 bits
• Host bits: 16 bits
• Default mask: 255.255.0.0

Class C addresses:

• Range: 192.0.0.0 - 223.255.255.255
• Network bits: 24 bits
• Host bits: 8 bits
• Default mask: 255.255.255.0

Class D addresses:

• Range: 224.0.0.0 - 239.255.255.255
• Purpose: multicast addresses

Class E addresses:

• Range: 240.0.0.0 - 255.255.255.255
• Purpose: reserved addresses

Special Addresses

• Network address: the host part is all zeros
• Broadcast address: the host part is all ones
• Loopback address: 127.0.0.0/8
• Private addresses:
  • Class A: 10.0.0.0/8
  • Class B: 172.16.0.0/12
  • Class C: 192.168.0.0/16

4. Subnetting

Subnet Masks

• Network part: the bits corresponding to 1s in the mask
• Host part: the bits corresponding to 0s in the mask
• CIDR notation: a slash followed by the number of network bits

Subnetting Steps

1. Determine the number of subnets: how many subnets are needed
2. Compute the number of network bits: 2^n ≥ the number of subnets
3. Determine the subnet mask: the mask that corresponds to the number of network bits
4. Compute the subnet addresses: the network address of each subnet
5. Determine the host range: the range of host addresses in each subnet

Example

Network: 192.168.1.0/24, split into 4 subnets

• Number of subnets: 4, which requires 2 network bits
• New mask: 255.255.255.192 (/26)
• Subnets:
  • 192.168.1.0/26 (0-63)
  • 192.168.1.64/26 (64-127)
  • 192.168.1.128/26 (128-191)
  • 192.168.1.192/26 (192-255)

5. CIDR (Classless Inter-Domain Routing)

Basic Concepts

• Classless addressing: the address space is no longer split into classes A, B, and C
• Flexible allocation: networks are divided as needed
• Smaller routing tables: entries are aggregated
• Higher efficiency: the routing table shrinks

Address Aggregation

• Prefix matching: addresses that share a prefix can be aggregated
• Longest match: the most specific route is chosen
• Fewer entries: several routing entries merge into one

6. Supporting Protocols

The ARP Protocol (Address Resolution Protocol)

Function: convert an IP address into a MAC address

How it works:

1. Send an ARP request packet
2. The target host replies with an ARP reply packet
3. Establish the mapping from IP address to MAC address
4. Cache the mapping information

The ARP table: stores the mapping from IP address to MAC address

The DHCP Protocol (Dynamic Host Configuration Protocol)

Function: assign IP addresses automatically

How it works:

1. DHCP Discover: the client broadcasts to find a DHCP server
2. DHCP Offer: the server offers an IP address
3. DHCP Request: the client requests to use the offered address
4. DHCP Acknowledge: the server confirms the address assignment

What is assigned:

• IP address
• Subnet mask
• Default gateway
• DNS servers

The ICMP Protocol (Internet Control Message Protocol)

Function: network diagnostics and error reporting

Message types:

• Echo request/reply: the ping command
• Destination unreachable: a routing failure
• Time exceeded: the TTL was exhausted
• Redirect: a better route exists

Applications:

• ping: connectivity testing
• traceroute: path tracing
• network diagnostics

Exercises

Exercise 1

What are the differences between IPv4 addresses and IPv6 addresses?

Exercise 2

What is CIDR, and what are its advantages?

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Glossary