CCNA

CIDR	Subnet Mask	Host Bits	Hosts
/32	255.255.255.255	0	1 (Host)
/31	255.255.255.254	1	0 (P2P)
/30	255.255.255.252	2	2
/29	255.255.255.248	3	6
/28	255.255.255.240	4	14
/27	255.255.255.224	5	30
/26	255.255.255.192	6	62
/25	255.255.255.128	7	126
/24	255.255.255.0	8	254
/23	255.255.254.0	9	510
/22	255.255.252.0	10	1,022
/21	255.255.248.0	11	2,046
/20	255.255.240.0	12	4,094
/19	255.255.224.0	13	8,190
/18	255.255.192.0	14	16,382
/17	255.255.128.0	15	32,766
/16	255.255.0.0	16	65,534
/15	255.254.0.0	17	131,070
/14	255.252.0.0	18	262,142
/13	255.248.0.0	19	524,286
/12	255.240.0.0	20	1,048,574
/11	255.224.0.0	21	2,097,150
/10	255.192.0.0	22	4,194,302
/9	255.128.0.0	23	8,388,606
/8	255.0.0.0	24	16,777,214

hub IPv4 Header: Every Field Explained

The IPv4 header is 20 bytes minimum (160 bits) and up to 60 bytes when options are present. It's organized in 32-bit (4-byte) words. Click any field or hex byte below to inspect.

0123 4567 891011 12131415 16171819 20212223 24252627 28293031

Version 4b

IHL 4b

DSCP 6b

ECN 2b

Total Length 16 bits

Identification 16 bits

Flags 3b

Fragment Offset 13 bits

Time to Live (TTL) 8 bits

Protocol 8 bits

Header Checksum 16 bits

Source IP Address 32 bits (4 bytes)

Destination IP Address 32 bits (4 bytes)

Options (if IHL > 5) 0 - 320 bits (Variable)

Padding Variable

code Example: Real Header in Hex (Click byte to decode)

45 Ver / IHL

00 DSCP / ECN

00 3C Length

1C 46 Ident

40 00 Flags / Off

40 TTL

06 Protocol

B1 E6 Checksum

C0 A8 00 68 Src IP

C0 A8 00 01 Dst IP

View Complete IPv4 Header Summary Table & Key Takeaways

Field	Size	Purpose
Version	4 bits	IP version (always 4)
IHL	4 bits	Header length in 32-bit words (IHL × 4 bytes)
DSCP	6 bits	Traffic class / QoS priority (RFC 2474)
ECN	2 bits	Explicit Congestion Notification (RFC 3168)
Total Length	16 bits	Header + data size in bytes (20 to 65,535 bytes)
Identification	16 bits	Groups fragments of one datagram together
Flags	3 bits	Fragmentation control: Reserved (0), DF (Don't Fragment), MF (More Fragments)
Fragment Offset	13 bits	Position of fragment in original packet in 8-byte units
Time to Live (TTL)	8 bits	Hop limit counter decremented by 1 at each router (loop prevention)
Protocol	8 bits	Upper-layer Layer 4 protocol (1 ICMP, 6 TCP, 17 UDP, 89 OSPF, etc.)
Header Checksum	16 bits	Header error detection (recomputed at every router hop)
Source Address	32 bits	Sender IPv4 address (rewritten by NAT)
Destination Address	32 bits	Receiver IPv4 address (used for router forwarding decisions)
Options	0-320 bits	Optional extras (Record Route, Timestamp, Source Route, Router Alert)

CCNA 200-301 Exam Takeaways:

Header size: Minimum 20 bytes (IHL = 5); maximum 60 bytes with options (IHL = 15).
Loop prevention: TTL decrements by 1 at every Layer 3 hop; when TTL = 0, the packet is dropped and ICMP Time Exceeded (Type 11) is returned. Traceroute relies on this behavior.
Must-know protocol numbers: 1 (ICMP), 6 (TCP), 17 (UDP), and 89 (OSPF).
Fragmentation: Controlled by Identification (groups fragments), Flags (DF = Don't Fragment / PMTUD; MF = More Fragments), and Fragment Offset (measured in 8-byte units).
Checksum: Protects the IPv4 header only (not the data payload) and must be recalculated at every router hop because TTL is decremented.
Addressing: IP addresses remain constant end-to-end (unless modified by NAT), while MAC addresses change hop-by-hop at every Layer 3 boundary.

settings_ethernet Ethernet Header: Every Field Explained

The standard LAN framing format is Ethernet II. Its header is 14 bytes (Destination MAC, Source MAC, and EtherType), expanding to 18 bytes on trunk links when an 802.1Q VLAN tag is added. Total frame size ranges from 64 to 1,518 bytes (1,522 bytes with 802.1Q). Click any field or hex byte to inspect.

Preamble 7 Bytes (56b)

SFD 1B

Destination MAC 6 Bytes (48 bits)

Source MAC 6 Bytes (48 bits)

EtherType 2 Bytes

Payload (L3 Data) 46 - 1,500 Bytes

FCS 4 Bytes

code Example: Real Frame in Hex (Click byte to decode)

00 1A 2B 3C 4D 5E Dest MAC

08 00 27 AB CD EF Src MAC

08 00 EtherType (IPv4)

45 00 00 3C ... Payload Data

View Complete Ethernet Summary, Frame Sizes & LLC/SNAP Format

Measurement	Minimum	Maximum	Notes
Payload (MTU)	46 Bytes (42 if tagged)	1,500 Bytes	Standard MTU (Jumbo frames up to ~9,000B in data centers)
Frame (Dest MAC → FCS)	64 Bytes	1,518 Bytes	Standard untagged Ethernet II frame
Frame with 802.1Q Tag	64 Bytes	1,522 Bytes	4-byte tag added on trunk links (TPID + TCI)
On the Wire (with Preamble, SFD, IFG)	84 Bytes	1,538 Bytes	Includes 7B preamble, 1B SFD, and 12B interframe gap

CCNA 200-301 Exam Takeaways:

Header size: Standard Ethernet II header is 14 bytes (6B Dest MAC + 6B Src MAC + 2B EtherType), expanding to 18 bytes with an 802.1Q VLAN tag.
Frame size boundaries: Valid frames range from 64 to 1,518 bytes (1,522 bytes with 802.1Q). Frames < 64 bytes are runts; frames > 1,518/1,522 bytes without jumbo frames are giants.
MAC Address format: 48 bits (6 bytes). First 24 bits = OUI (vendor code); last 24 bits = vendor-assigned device ID.
Switch operation: Switches learn MAC addresses from the Source MAC of incoming frames to build the CAM table; switches forward based on the Destination MAC.
Must-know EtherTypes: 0x0800 (IPv4), 0x0806 (ARP), 0x86DD (IPv6), and 0x8100 (802.1Q Tag).
Error detection: The 4-byte FCS (CRC-32) trailer detects frame corruption. Corrupted frames are silently dropped without retransmission (Layer 4 TCP handles reliability).

lan CCNA 200-301 Ports & IP Protocol Numbers

Identifier	Protocol	Header Type	CCNA 200-301 Exam Context
Protocol 1	ICMP	IP Protocol (L3)	Internet Control Message Protocol (Ping / Traceroute)
Protocol 6	TCP	IP Protocol (L3)	Transmission Control Protocol (Reliable, Connection-Oriented)
Protocol 17	UDP	IP Protocol (L3)	User Datagram Protocol (Unreliable, Connectionless)
Protocol 47	GRE	IP Protocol (L3)	Generic Routing Encapsulation (Site-to-Site VPN Tunnels)
Protocol 50	IPsec ESP	IP Protocol (L3)	Encapsulating Security Payload (IPsec Data Encryption & Auth)
Protocol 51	IPsec AH	IP Protocol (L3)	Authentication Header (IPsec Data Integrity & Auth)
Protocol 58	ICMPv6	IPv6 Next Header	IPv6 Neighbor Discovery (NDP), Router Solicit/Advert (RS/RA)
Protocol 88	EIGRP	IP Protocol (L3)	Enhanced Interior Gateway Routing Protocol (Cisco)
Protocol 89	OSPF	IP Protocol (L3)	Open Shortest Path First (OSPFv2 & OSPFv3)
Port 20, 21	FTP	TCP Port (L4)	File Transfer Protocol (Data: 20, Control: 21)
Port 22	SSH	TCP Port (L4)	Secure Shell (Encrypted Remote CLI Access)
Port 23	Telnet	TCP Port (L4)	Unencrypted Remote CLI Access (Plaintext)
Port 25	SMTP	TCP Port (L4)	Simple Mail Transfer Protocol (Email Sending)
Port 49	TACACS+	TCP Port (L4)	Cisco AAA Architecture (Encrypts Entire Packet Payload)
Port 53	DNS	TCP / UDP Port (L4)	Domain Name System (UDP Queries, TCP Zone Transfers)
Port 67, 68	DHCP	UDP Port (L4)	Dynamic Host Config Protocol (Server: 67, Client: 68)
Port 69	TFTP	UDP Port (L4)	Trivial File Transfer Protocol (Unreliable, No Auth)
Port 80	HTTP	TCP Port (L4)	Hypertext Transfer Protocol (Unencrypted Web)
Port 123	NTP	UDP Port (L4)	Network Time Protocol (Clock Sync across Network)
Port 161, 162	SNMP	UDP Port (L4)	Simple Network Mgmt (Agent Poll: 161, Traps: 162)
Port 179	BGP	TCP Port (L4)	Border Gateway Protocol (Path-Vector Exterior Routing)
Port 443	HTTPS	TCP Port (L4)	HTTP Secure (Encrypted Web Traffic via TLS/SSL)
Port 514	Syslog	UDP Port (L4)	System Logging (Default Remote Log Server Port)
Port 1812, 1813	RADIUS	UDP Port (L4)	Open AAA Standard (Auth/Authz: 1812, Accounting: 1813)

alt_route Administrative Distances (AD) & Route Preference

Route Source / Protocol	AD Value	Default Status & Context
Connected Interface	0	Highest possible trustworthiness
Static Route	1	Default static route preference
EIGRP Summary Route	5	Auto or manual summary route to Null0
External BGP (eBGP)	20	Routes learned from external Autonomous System
Internal EIGRP	90	Normal EIGRP routes within same AS
OSPF	110	Open Shortest Path First (Inter-area, Intra-area, External)
IS-IS	115	Intermediate System to Intermediate System
RIP	120	Routing Information Protocol (v1 / v2)
External EIGRP	170	Redistributed routes into EIGRP
Internal BGP (iBGP)	200	Routes learned via BGP within same AS
DHCP Default Route	254	Default route assigned via DHCP client
Unusable / Untrusted	255	Route will not be installed in Routing Table (RIB)

public IPv6 Special & Multicast Addresses

Prefix / Address	Address Type	Description & Scope
::/128	Unspecified	Used as source address before host receives IP assignment
::1/128	Loopback	Local host loopback testing (equivalent to 127.0.0.1)
2000::/3	Global Unicast (GUA)	Globally routable public IPv6 range (2000:: to 3FFF::)
fe80::/10	Link-Local	Auto-configured on every IPv6 interface (Non-routable)
fc00::/7 (fd00::/8)	Unique Local (ULA)	Private IPv6 addressing (equivalent to RFC 1918 IPv4)
ff00::/8	Multicast Range	All IPv6 multicast destination addresses
ff02::1	Multicast	All IPv6 Nodes on local link (Replaces IPv4 Broadcast)
ff02::2	Multicast	All IPv6 Routers on local link
ff02::5	Multicast	All OSPFv3 Routers
ff02::6	Multicast	OSPFv3 Designated Routers (DR / BDR)
ff02::9	Multicast	All RIPng Routers
ff02::a	Multicast	All EIGRPv6 Routers
ff02::1:ff00:0/104	Solicited-Node	Used for ARP replacement (NDP Neighbor Solicitation)

memory Cisco Memory Architecture & Ethernet Headers

Component / Header	Category	Contents & Details
RAM	Volatile Memory	Running-config, ARP cache, Routing table (RIB), MAC table, Buffers
NVRAM	Non-Volatile Memory	Startup-config file, Configuration register setting
Flash Memory	Non-Volatile Memory	Cisco IOS operating system image (.bin file), vlan.dat file
ROM	Non-Volatile Memory	POST (Power-On Self Test), Bootstrap program, ROMmon recovery mode
0x2102	Config Register	Normal default boot process (Loads IOS from Flash, startup-config from NVRAM)
0x2142	Config Register	Bypasses NVRAM startup-config on boot (Used for Password Recovery!)
EtherType 0x0800	Ethernet Frame	IPv4 Data Packet Payload
EtherType 0x0806	Ethernet Frame	ARP (Address Resolution Protocol) Frame Payload
EtherType 0x86DD	Ethernet Frame	IPv6 Data Packet Payload
EtherType 0x8100	Ethernet Frame	802.1Q VLAN Tagged Frame Header
MAC 01-00-5E-xx-xx-xx	Multicast MAC	IPv4 Multicast Destination MAC (01-00-5E + 23 bits of IPv4 Multicast IP)
MAC 33-33-xx-xx-xx-xx	Multicast MAC	IPv6 Multicast Destination MAC (33-33 + last 32 bits of IPv6 Multicast IP)

account_tree Spanning Tree (STP) & Wireless (802.11) Standards

Standard / Concept	Type / Frequency	Key Details & CCNA Specifications
802.1D (Classic STP)	Legacy STP	Port states: Blocking -> Listening (15s) -> Learning (15s) -> Forwarding. Hello: 2s, Max Age: 20s
802.1w (RSTP)	Rapid STP	Port states: Discarding -> Learning -> Forwarding. Fast convergence via proposal/agreement handshake
802.1s (MSTP)	Multiple STP	Maps multiple VLANs to STP instances to save CPU resources
802.11b	Wi-Fi 1 (2.4 GHz)	Max Speed: 11 Mbps (DSSS modulation, non-overlapping channels 1, 6, 11)
802.11a	Wi-Fi 2 (5 GHz)	Max Speed: 54 Mbps (OFDM modulation)
802.11g	Wi-Fi 3 (2.4 GHz)	Max Speed: 54 Mbps (Backward compatible with 802.11b)
802.11n	Wi-Fi 4 (2.4 & 5 GHz)	Max Speed: 600 Mbps (Introduced MIMO & Channel Bonding 40MHz)
802.11ac	Wi-Fi 5 (5 GHz)	Max Speed: 6.9 Gbps (MU-MIMO, 80/160MHz Channels, 256-QAM)
802.11ax	Wi-Fi 6 / 6E (2.4/5/6GHz)	Max Speed: 9.6 Gbps (OFDMA, 1024-QAM, Target Wake Time)

CCNA

Subnet Cheat Sheet

Jeremy's IT Lab Course Progress

hub IPv4 Header: Every Field Explained

settings_ethernet Ethernet Header: Every Field Explained

lan CCNA 200-301 Ports & IP Protocol Numbers

alt_route Administrative Distances (AD) & Route Preference

public IPv6 Special & Multicast Addresses

memory Cisco Memory Architecture & Ethernet Headers

account_tree Spanning Tree (STP) & Wireless (802.11) Standards

science CCNA Mega Lab

Day Title

Quiz Completed!

1. Networking Devices