CCNA 200-301: Network Fundamentals (20%) — Cheat Sheet

OSI Model & TCP/IP Stack

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Key: Each layer adds header = encapsulation; remove on receive = decapsulation.

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IPv4 Addressing & Subnetting (MOST CRITICAL)

Binary-to-Decimal & Decimal-to-Binary

Powers of 2: 128, 64, 32, 16, 8, 4, 2, 1 (left-to-right)

Each octet: 0–255

Subnet Mask to CIDR (Block-Size Method)

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Block-Size Formula: 256 ÷ (256 − last octet of mask) = block size

Subnet Calculation:

Increment address by block size

First subnet = network; last = broadcast

Usable host range = network + 1 to broadcast − 1

Wildcard Mask

Inverse of subnet: 255.255.255.0 subnet = 0.0.0.255 wildcard

Used in: ACLs, OSPF network statements, route filtering

IPv4 Address Classes (Reference Only — Deprecated)

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TCP vs. UDP

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TCP Three-Way Handshake:

Client → Server: SYN (seq=x)

Server → Client: SYN-ACK (seq=y, ack=x+1)

Client → Server: ACK (seq=x+1, ack=y+1)

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Common Ports & Protocols

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Switching Fundamentals

MAC Learning & Forwarding

CAM Table: Switches store MAC → port mappings (source MAC learned on ingress)

Forwarding Logic:

- Known unicast: forward to learned port - Unknown unicast: flood all ports except ingress - Broadcast/multicast: flood all ports except ingress - Implicit deny: if not learned, drop

Duplex Modes

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4	Transport	Transport	Segment	TCP, UDP
Layer	OSI Name	TCP/IP	PDU	Key Protocols
7	Application	Application	Data	HTTP, HTTPS, DNS, DHCP, Telnet, SSH, FTP, TFTP
6	Presentation	–	Data	Encryption, compression, translation
5	Session	–	Data	Session management, dialogue control
3	Network	Internet	Packet	IP (v4/v6), ICMP, IGMP
2	Data Link	Link	Frame	Ethernet, 802.1Q, MAC, ARP
1	Physical	–	Bit	Copper, fiber, RJ-45, SFP
Mask	CIDR	Block Size	Hosts/Subnet
255.255.255.0	/24	256	254
255.255.255.128	/25	128	126
255.255.255.192	/26	64	62
255.255.255.224	/27	32	30
255.255.255.240	/28	16	14
255.255.255.248	/29	8	6
255.255.255.252	/30	4	2
255.255.255.0	/24	256	254
Class	Range	Default Mask	Private Range
A	1–126.x.x.x	/8	10.0.0.0/8
B	128–191.x.x	/16	172.16.0.0/12
C	192–223.x.x	/24	192.168.0.0/16
Feature	TCP	UDP
Connection	3-way handshake (SYN, SYN-ACK, ACK)	Connectionless
Reliability	Guaranteed delivery; retransmission	Best effort (lossy)
Ordering	In-order delivery	No order guarantee
Speed	Slower (overhead)	Fast
Flow Control	Windowing (sliding window)	None
Use Cases	HTTP, HTTPS, SSH, FTP, SMTP, DNS (zone transfer)	DNS queries, DHCP, NTP, TFTP, SNMP, VoIP
Port Examples	80, 443, 22, 21, 25, 53	53, 67, 68, 69, 123, 161
Port(s)	Protocol	Transport
20, 21	FTP	TCP
22	SSH	TCP
23	Telnet	TCP
25	SMTP	TCP
53	DNS	TCP/UDP
67, 68	DHCP	UDP
69	TFTP	UDP
80	HTTP	TCP
110	POP3	TCP
123	NTP	UDP
143	IMAP	TCP
161	SNMP Get/Inform	UDP
162	SNMP Trap	UDP
443	HTTPS	TCP
Mode	Description	Collision
Half-duplex	Send or receive (not both)	Can collide (CSMA/CD)
Full-duplex	Send and receive simultaneously

Network Fundamentals (20%)