VLSM — Variable Length Subnet Masking
VLSM lets you use different subnet masks inside the same network, so IP space is utilized efficiently. Facts / key topics RFC: not a standalone RFC – part of the CIDR concept Layer 3 Requires classless routing protocols (OSPF, EIGRP, RIPv2, IS-IS, BGP) Does not work with classful routing (RIPv1, IGRP) Used in: enterprise LAN/WAN, […]
Special IPv4 Addresses
Special IPv4 addresses are reserved for specific purposes and cannot be used as normal host addresses. Network address Identifies the subnet itself – cannot be assigned to a host All host bits = 0 Example: 192.168.1.0/24 Broadcast address Sends traffic to all hosts in the subnet All host bits = 1 Example: 192.168.1.255/24 Loopback Tests […]
Private vs Public IP Addresses
IPv4 addressing gives every device a unique 32-bit address so packets can be identified and delivered correctly. Facts / key topics Standard: RFC 791 · Length: 32 bits · Notation: dotted decimal (e.g. 192.168.1.1) · OSI: Layer 3 Address types: unicast, broadcast, multicast CCNA: understand + subnetting + calculations · CCNP: design, summarization, edge cases […]
The TCP/IP Model (Cisco 5-Layer)
The Cisco TCP/IP model has 5 layers – the Network Access layer is split into Data Link and Physical. Layer Name Primary function Scope Typical protocols Data unit 5 Application Network services for applications Application-to-application HTTP, HTTPS, FTP, SMTP, DNS, DHCP, SNMP, SSH, NTP Messages 4 Transport End-to-end transport, ports, reliability End-to-end / host-to-host TCP, […]
The OSI Model — Layers, Scopes, Protocols
OSI model – functions, scope and examples Layer Name Primary function Scope Typical protocols Data unit 7 Application Delivers network services directly to user/system applications Application-to-application HTTP, FTP, SMTP, DNS, DHCP, SNMP, NTP, SSH Messages, files, commands 6 Presentation Formats data: encoding, encryption, decryption, compression End-to-end (logical) TLS, SSL, JPEG, PNG, ASCII, MPEG Data streams, […]
The Ethernet II Frame
The Ethernet II frame (with the physical-layer preamble/SFD) is built as follows: PREAMBLE START FRAME DELIMITER DESTINATION MAC SOURCE MAC LENGTH OR TYPE DATA FRAME CHECK SEQUENCE 7 bytes 1 byte 6 bytes 6 bytes 2 bytes 46–1500 bytes 4 bytes Preamble: syncs the NIC with the incoming bits Start Frame Delimiter (SFD): tells the […]
The IPv6 Header
The IPv6 header is a fixed 40-byte Layer 3 header designed for faster routing, better scalability and flexible extensions via Extension Headers (RFC 8200). Facts / key topics Standard / RFC: RFC 8200 · OSI layer: Layer 3 Header length: fixed 40 bytes (no options inside the header) Checksum: ❌ none (removed for performance) · […]
The IPv4 Header
The IPv4 header is a variable Layer 3 header (RFC 791, 20–60 bytes) containing everything a router uses to forward, fragment and deliver IPv4 packets. Facts / key topics Standard / RFC: RFC 791 · OSI layer: Layer 3 (Network) Header length: 20–60 bytes (20 bytes without options) Checksum: ✔ yes (header only) · Broadcast: […]
Encapsulation & Decapsulation
Encapsulation is the process where data is wrapped layer by layer with headers (and sometimes trailers) so it can be transmitted across the network; decapsulation is the reverse process on receipt. Facts / key topics OSI layers: L7 → L1 (encapsulation), L1 → L7 (decapsulation) Data units: L7–L5: Data · L4: Segment (TCP) / Datagram […]
Collision vs Broadcast Domains
Collision domain: where frames can collide (only relevant for half-duplex). Broadcast domain: where broadcasts (ff:ff:ff:ff:ff:ff) are forwarded. Collision domain What is it? A network segment where two devices can transmit at the same time and cause collisions. OSI layer: L1/L2 Relevant technology: Ethernet (CSMA/CD – historical) How is it divided? Hub → one large collision […]