Internet Protocol Version 4 (IPv4)

IPv4 is the foundational Layer 3 protocol of the TCP/IP stack, responsible for logical addressing, routing, and packet delivery across networks. Although its address space is limited, it remains widely deployed across global networks.

Category

Description

Use Case

IPv4 Basics

Covers the core format, header structure, and key attributes of IPv4. 32-bit address space, connectionless operation, TTL, fragmentation.

LAN/WAN connectivity, internet communication

IPv4 Header Structure

Explains the layout and fields of the IPv4 packet header. Includes version, header length, total length, protocol, source/destination IPs.

Packet parsing, firewall rules, network diagnostics

IPv4 Packet Routing

Covers how IPv4 packets are forwarded across networks using routing tables. Path selection based on destination IP and next-hop logic.

Internet communication, multi-network environments, router configuration

IPv4 Fragmentation

Describes how large IPv4 packets are split into smaller fragments to traverse networks with lower MTUs. Includes identification, flags, fragment offset.

Transmission over heterogeneous networks, avoiding packet drops due to MTU limits

IPv4 Connectionless Protocol

Explains that IPv4 is a connectionless protocol, meaning packets are routed independently without session state. Each datagram is treated separately.

Stateless communication, DNS queries, video streaming, VoIP

IPv4 Protocol Identification

Details the Protocol field in the IPv4 header that specifies the upper-layer protocol (e.g., TCP, UDP, ICMP). Enables correct payload handling at the destination.

Packet demultiplexing, firewall filtering, protocol-specific handling

IPv4 Time to Live (TTL)

Describes the TTL field in the IPv4 header which limits the lifespan of packets to prevent infinite looping. Decremented at each router hop.

Loop prevention, traceroute, path diagnostics

IPv4 Addressing

How IPv4 addresses are structured, assigned, and translated. Unicast, multicast, broadcast, NAT, private ranges, CIDR.

Host identification, subnetting, address conservation

Anycast

One-to-nearest delivery. Packets go to the closest destination with the shared IP.

Load balancing, DNS, CDN nodes.

Broadcast

One-to-all delivery within a subnet. IPv4 supports this.

ARP, DHCP discovery.

CIDR

Classless subnetting with VLSM. Replaces classful addressing.

Route aggregation, modern IP allocation.

Classful

Legacy method using fixed IP blocks (A, B, C, etc.).

Historical networks.

Multicast

One-to-many group communication.

Streaming, OSPF, conferencing.

NAT

Private-to-public IP mapping.

IPv4 conservation, home routers.

Subnetting

Divides networks into smaller subnets.

IP management, routing control.

Unicast

One-to-one delivery.

Browsing, file transfers, email.

EGP (Exterior Gateway Protocol)

One of the earliest routing protocols used to exchange routing info between autonomous systems. Largely obsolete and replaced by BGP.

Inter-AS routing in early networks.

EIGRP (Enhanced Interior Gateway Routing Protocol)

Cisco proprietary protocol combining features of distance-vector and link-state protocols. Supports VLSM, fast convergence, and uses DUAL algorithm.

Efficient routing within Cisco-based enterprise networks.

IGRP (Interior Gateway Routing Protocol)

Older Cisco proprietary protocol, now deprecated in favor of EIGRP. Limited scalability and slow convergence.

Legacy enterprise routing.

IS-IS (Intermediate System to Intermediate System)

Link-state protocol designed for large ISP networks. Scales well and supports both IPv4 and IPv6.

Core ISP and enterprise backbone routing.

Multiprotocol BGP (MP-BGP)

Extension of BGP that supports routing for multiple protocols, including IPv6 and MPLS VPNs.

Multi-protocol environments and MPLS networks.

OSPF (Open Shortest Path First)

Open standard link-state protocol using Dijkstra’s algorithm. Supports areas, fast convergence, and VLSM.

Hierarchical and scalable enterprise routing.

RIPv1 (Routing Information Protocol v1)

Early distance-vector protocol using hop count metric. No support for CIDR or VLSM.

Small networks or legacy equipment.

RIPv2 (Routing Information Protocol v2)

An enhanced version of RIPv1, supporting authentication, CIDR, and multicast updates.

Small-to-medium networks with basic routing needs.

RFC: RFC 791

Main Features:

  • 32-bit addressing scheme in dotted-decimal format (e.g., 192.168.0.1)

  • Operates at OSI Layer 3 (Network Layer)

  • Provides logical addressing and packet fragmentation

  • Header includes TTL, checksum, protocol, and source/destination IP

  • Stateless and connectionless: each packet is treated independently

  • Widely deployed across all modern networks

Use Cases:

  • Core of internet communication

  • IP routing within and between enterprise networks

  • Host identification in TCP/IP model

Alternative Protocols:

  • IPv6 – Next-generation protocol with larger address space and security

  • IPX – Legacy Novell protocol (obsolete)

  • AppleTalk – Deprecated Apple protocol

Let us learn more about IPv4:

Jump to “IPv4 Basics”

RFC: RFC 791

Main Features:

  • Fixed 20-byte base header with optional fields

  • Fields include version, IHL, total length, TTL, protocol, checksum, source & destination IP

  • Supports fragmentation with Identification, Flags, and Fragment Offset fields

  • Uses Protocol field to indicate upper-layer (TCP, UDP, etc.)

  • TTL limits packet lifetime and prevents infinite routing loops

  • Checksum ensures integrity of the IPv4 header

Use Cases:

  • Packet parsing and inspection by firewalls and routers

  • Deep packet inspection in IDS/IPS systems

  • Teaching core structure of IP networking

  • Troubleshooting with tools like Wireshark

Alternative Protocols:

  • IPv6 – Simplified header with fixed length and no fragmentation

  • MPLS – Used in high-performance networks as an alternative to traditional IP routing

  • GRE – Tunneling protocol that wraps IP headers for encapsulated routing

Let us learn more about IPv4 Header Structure:

Jump to “IPv4 Header Structure”

RFC: RFC 791, RFC 1812

Main Features:

  • Routing uses destination IP address and routing tables to forward packets

  • Decisions are made hop-by-hop, not end-to-end

  • Routers use static or dynamic routing protocols (e.g., RIP, OSPF, BGP)

  • Longest prefix match algorithm determines best next hop

  • TTL is decremented at each router hop to avoid loops

  • Packets may follow different paths (asymmetric routing)

Use Cases:

  • Forwarding IP packets across multiple networks

  • Enterprise network design and internet backbone routing

  • Dynamic route learning and failover handling

  • Implementation of routing policies (e.g., QoS, access control)

Alternative Protocols:

  • MPLS – Label-based routing used in service provider networks

  • IPv6 Routing – Similar logic but with improved features

  • SDN – Centralized routing control (e.g., OpenFlow)

Let us learn more about IPv4 Packet Routing:

Jump to “IPv4 Packet Routing”

RFC: RFC 791

Main Features:

  • Allows large IP packets to be broken into smaller fragments for networks with smaller MTUs

  • Uses Identification, Flags, and Fragment Offset fields in the IPv4 header

  • Fragments are reassembled only at the destination host

  • Routers do not perform reassembly

  • If a single fragment is lost, the entire datagram is discarded

Use Cases:

  • Transmitting large packets over mixed-MTU networks

  • Interoperability with older or constrained hardware

  • Understanding MTU, path MTU discovery, and ICMP Fragmentation Needed messages

Alternative Protocols:

  • IPv6 – Does not allow routers to fragment packets; fragmentation is host-controlled

  • GRE/IPSec – Often requires manual fragmentation handling

  • Path MTU Discovery (PMTUD) – Reduces need for fragmentation

Let us learn more about IPv4 Fragmentation:

Jump to “IPv4 Fragmentation”

RFC: RFC 791

Main Features:

  • IPv4 is a connectionless protocol: it does not establish a session before sending data

  • Each datagram is routed independently and may take different paths

  • No guarantees for delivery, ordering, or duplication prevention

  • Lightweight and efficient for many applications

Use Cases:

  • Stateless communication like DNS, VoIP, video streaming

  • Protocols that build their own reliability (e.g., TCP)

  • Systems that prioritize performance over guaranteed delivery

Alternative Protocols:

  • IPv6 – Also connectionless

  • TCP – Adds connection-oriented behavior on top of IP

  • SCTP – Combines connectionless and connection-oriented properties

Let us learn more about IPv4 as a Connectionless Protocol:

Jump to “IPv4 Connectionless Protocol”

RFC: RFC 791

Main Features:

  • Uses the “Protocol” field in the IPv4 header to indicate the encapsulated transport-layer protocol

  • Helps the destination system process the packet correctly (e.g., TCP, UDP, ICMP)

  • 8-bit field supports up to 256 different protocol types

  • Common values: 6 (TCP), 17 (UDP), 1 (ICMP)

Use Cases:

  • Packet inspection and protocol filtering in firewalls

  • Enabling routers and hosts to properly forward packets to the correct layer

  • Deep packet inspection in security tools

Alternative Protocols:

  • IPv6 – Uses “Next Header” field with similar function

  • Ethernet Type Field – Similar functionality at data link layer

  • SCTP – Encapsulates multiple protocols with built-in multiplexing

Let us learn more about IPv4 Protocol Identification:

Jump to “IPv4 Protocol Identification”

RFC: RFC 791

Main Features:

  • TTL field is an 8-bit value in the IPv4 header

  • Limits the lifespan of a packet by specifying the number of hops allowed

  • Decremented by 1 at each router hop; discarded when TTL reaches 0

  • Prevents infinite loops caused by routing misconfigurations

Use Cases:

  • Loop prevention in IP routing

  • Diagnostics using traceroute and TTL expiration

  • Detecting network topology or delays

Alternative Protocols:

  • IPv6 – Replaces TTL with “Hop Limit” field, serving the same purpose

  • ICMP – Used in conjunction with TTL for Time Exceeded messages

  • MPLS TTL – Similar mechanism in label-switched paths

Let us learn more about IPv4 Time To Live (TTL):

Jump to “IPv4 Time To Live (TTL)”

RFCs: RFC 791, RFC 950, RFC 1918, RFC 3022

Main Features:

  • 32-bit address space with ~4.3 billion addresses

  • Address Types: - Unicast: one-to-one - Broadcast: one-to-all on subnet (255.255.255.255) - Multicast: Class D (224.0.0.0 – 239.255.255.255)

  • CIDR (Classless Inter-Domain Routing): /n format for flexible subnetting

  • Private Address Ranges: - 10.0.0.0/8 - 172.16.0.0/12 - 192.168.0.0/16

  • NAT: Translates private addresses to public for internet access

Use Cases:

  • Host IP allocation in enterprise and ISP networks

  • Internet access via NAT from private IPs

  • Network segmentation via subnetting

Alternative Protocols:

  • IPv6 – Uses 128-bit address space and does not require NAT

  • DHCP – For automatic IP address assignment

  • MAC – Layer 2 addressing (hardware level)

Let us learn more about IPv4 Addressing:

Jump to “IPv4 Addressing”

RFC: RFC 1546

Main Features:

  • One-to-nearest delivery based on routing metrics.

  • Multiple hosts share the same IP.

Use Cases:

  • DNS root servers

  • Content delivery networks (CDNs)

  • Load balancing in routing

Learn More About Anycast:

Jump to “Anycast”

RFC: RFC 919, RFC 922

Main Features:

  • One-to-all within subnet.

  • IPv4 only; IPv6 does not support broadcast.

Use Cases:

  • ARP discovery

  • DHCPDISCOVER messages

Learn More About Broadcast:

Jump to “Broadcast”

RFC: RFC 4632

Main Features:

  • Replaces classful addressing.

  • Allows subnetting with variable-length subnet masks.

Use Cases:

  • Route summarization

  • IP space efficiency in ISPs

Learn More About CIDR:

Jump to “CIDR”

RFC: RFC 791 (conceptual, now obsolete)

Main Features:

  • Divides address space into Class A, B, C, etc.

  • Lacks flexibility, poor address utilization.

Use Cases:

  • Legacy IP networks

  • Educational/historical references

Learn More About Classful Addressing:

Jump to “Classful”

RFC: RFC 1112, RFC 5771

Main Features:

  • One-to-many group delivery.

  • More efficient than broadcast for large-scale data.

Use Cases:

  • IPTV

  • Video conferencing

  • OSPF, PIM routing updates

Learn More About Multicast:

RFC: RFC 3022

Main Features:

  • Translates private IP to public.

  • Extends IPv4 usability and adds a security layer.

Use Cases:

  • Home/enterprise firewalls

  • ISPs managing limited IPv4 blocks

Learn More About NAT:

Jump to “NAT”

RFC: RFC 950

Main Features:

  • Divides larger IP space into smaller, manageable parts.

  • Enhances routing and security.

Use Cases:

  • Enterprise internal segmentation

  • ISP hierarchical IP design

Learn More About Subnetting:

Jump to “Subnetting”

RFC: RFC 791

Main Features:

  • One-to-one data transfer.

  • Most common form of IP communication.

Use Cases:

  • Web browsing

  • Emails, file sharing

Learn More About Unicast:

Jump to “Unicast”

RFC: RFC 904, RFC 1772 (Historic)

Main Features:

  • One of the earliest routing protocols used to exchange routing info between autonomous systems.

  • Largely obsolete and replaced by BGP.

Use Cases:

  • Inter-AS routing in early networks.

Alternative Protocols:

  • BGP – Modern exterior gateway protocol used globally.

Let us learn more about EGP:

Jump to “EGP”

RFC: RFC 1142, RFC 1195, RFC 5308 (IPv6 support)

Main Features:

  • Link-state protocol designed for large ISP networks.

  • Scales well and supports both IPv4 and IPv6.

Use Cases:

  • Core ISP and enterprise backbone routing.

Alternative Protocols:

  • OSPF – Another scalable link-state protocol.

  • BGP – For inter-domain routing.

Let us learn more about IS-IS:

Jump to “IS-IS”

RFC: RFC 4271 (BGP-4), RFC 4760 (Multiprotocol Extensions)

Main Features:

  • Extension of BGP that supports routing for multiple protocols, including IPv6 and MPLS VPNs.

Use Cases:

  • Multi-protocol environments and MPLS networks.

Alternative Protocols:

  • EIGRP – For internal routing within organizations.

  • IS-IS – Alternative backbone routing protocol.

Let us learn more about MP-BGP:

Jump to “MP-BGP”

RFC: RFC 2328 (OSPFv2)

Main Features:

  • Open standard link-state protocol using Dijkstra’s algorithm.

  • Supports areas, fast convergence, and VLSM.

Use Cases:

  • Hierarchical and scalable enterprise routing.

Alternative Protocols:

  • IS-IS – Another open standard link-state protocol.

  • EIGRP – Cisco proprietary alternative.

Let us learn more about OSPF:

Jump to “OSPF”

RFC: RFC 1058

Main Features:

  • Early distance-vector protocol using hop count metric.

  • No support for CIDR or VLSM.

Use Cases:

  • Small networks or legacy equipment.

Alternative Protocols:

  • RIPv2 – Improved version with modern features.

  • OSPF – For larger or more complex networks.

Let us learn more about RIPv1:

Jump to “RIPv1”

RFC: RFC 2453

Main Features:

  • An enhanced version of RIPv1, supporting authentication, CIDR, and multicast updates.

Use Cases:

  • Small-to-medium networks with basic routing needs.

Alternative Protocols:

  • OSPF – More scalable and feature-rich.

  • EIGRP – Cisco proprietary alternative.

Let us learn more about RIPv2:

Jump to “RIPv2”