KNOW THESE NET +

ping

Sends ICMP echo requests to test whether a host is reachable and measure round-trip time

traceroute or tracert

Traces the path packets take to a destination, showing each router hop and its response time. tracert on Windows, traceroute on Linux

pathping

Windows tool combining ping and tracert, showing packet loss at each hop over time

nslookup

Queries DNS servers to resolve a hostname to an address or an address back to a name

dig

Domain Information Groper. Linux DNS query tool giving detailed record information for diagnosing resolution problems

tcpdump

Command-line packet analyzer on Linux that captures and filters traffic on an interface

netstat

Displays active connections, listening ports, and the routing table on the local machine

ipconfig

Windows command showing IP configuration. Use /all for full detail, /release and /renew for DHCP, /flushdns to clear the DNS cache

ifconfig

Older Linux command to display and configure network interfaces and addresses

ip

Modern Linux command to display and configure network interfaces, addresses, and routes

arp

Displays or modifies the table mapping IP addresses to MAC addresses on the local network. Use arp -a to list it

nmap

Network Mapper. Scans the network to discover hosts and identify open ports and services

show mac-address-table

Displays which MAC addresses the switch has learned and on which ports, used to locate a device

show route

Displays the routing table, listing known networks and the next hop for each

show interface

Displays an interface's status, speed, duplex, and error counters such as CRC errors, runts, giants, and drops

show config

Displays the device's current running configuration

show arp

Displays the ARP table mapping IP addresses to MAC addresses

show vlan

Displays configured VLANs and which ports belong to each

show power

Displays PoE power budget, allocation, and per-port consumption

802.3

Wired Ethernet standards, defining frame formats and physical layer specifications for copper and fiber

802.3af

PoE. Supplies up to 15.4W per port

802.3at

PoE+. Supplies up to 30W per port

802.3bt

PoE++. Supplies up to 60W or 90W per port

802.3ad

Link aggregation, implemented by LACP, bundling multiple links into one logical link

802.1Q

VLAN tagging standard that inserts a VLAN ID into frames so one trunk carries multiple VLANs

802.1D

The original Spanning Tree Protocol, preventing Layer 2 loops by blocking redundant paths

802.1w

Rapid Spanning Tree Protocol, converging faster than the original STP

802.1X

Port-based network access control, using EAP to authenticate a device before granting network access

802.11

Wireless LAN standards, known as Wi-Fi

802.11a

5 GHz, up to 54 Mbps. Less interference than 2.4 GHz but shorter range

802.11b

2.4 GHz, up to 11 Mbps. Longer range and better penetration, but more interference

802.11g

2.4 GHz, up to 54 Mbps, backward compatible with 802.11b

802.11n

Wi-Fi 4. 2.4 and 5 GHz, up to 600 Mbps, using MIMO with multiple antennas

802.11ac

Wi-Fi 5. 5 GHz only, with wider channels and more spatial streams for multi-gigabit speeds

802.11ax

Wi-Fi 6. 2.4 and 5 GHz, up to about 9.6 Gbps. Adds OFDMA and BSS coloring for crowded areas

802.11h

Adds dynamic frequency selection and transmit power control to 5 GHz so Wi-Fi avoids radar interference

IEEE 1588

Precision Time Protocol, delivering microsecond-level clock synchronization

Mesh topology

Nodes connect directly to many others with no central point, so traffic reroutes around a failed link

Hybrid topology

Blends two or more topologies to gain their strengths and offset their weaknesses

Star or hub and spoke

Every node connects to one central device. Simple to manage but a single point of failure

Spine and leaf

Every leaf switch connects to every spine switch, so any two devices are the same short distance apart. Used in data centers

Point-to-point

A direct link between exactly two devices, used for dedicated connections

Three-tier hierarchical model

Network design split into core, distribution, and access layers

Core layer

The backbone, built for fast high-volume switching with heavy redundancy and no policy work

Distribution layer

Sits between core and access, handling routing, filtering, and aggregating access switches

Access layer

The entry point where end devices connect, made of switches and access points

Collapsed core

Merges the core and distribution layers into one, cutting cost for small and medium networks

North-south traffic

Flows between the data center and the outside world, typically clients reaching hosted services

East-west traffic

Flows inside the data center between servers, storage, and virtual machines

Network loop

Redundant Layer 2 paths with no loop prevention, so frames circle endlessly and flood the network

Root bridge

The central reference switch in STP. The lowest bridge ID wins, comparing priority first and MAC address as the tiebreaker

Root port

The port on a non-root switch with the lowest path cost back to the root bridge

Designated port

The forwarding port chosen for each network segment as the best path toward the root

Alternate port

A backup path to the root that stays blocked until the primary path fails. Normally sits in blocking

Blocking state

The port forwards no user traffic and learns no MAC addresses, but still listens for BPDUs

Listening state

The port exchanges BPDUs to take part in the topology calculation but doesn't learn MAC addresses or forward data

Learning state

The port builds its MAC address table but still doesn't forward user traffic

Forwarding state

The port is fully active, learning addresses and passing user traffic

STP port state order

Blocking, Listening, Learning, Forwarding

Error disabled

A port the switch shut down automatically after detecting a violation such as a port security breach or a loop

Administratively down

A port a technician deliberately disabled with the shutdown command

Suspended

A port temporarily held out of service by a mismatch, often a link aggregation member that doesn't match the bundle

BPDU

Bridge Protocol Data Unit. The messages switches exchange to build and maintain the spanning tree

Collision domain

A segment where collisions can occur. Each switch port is its own collision domain

Broadcast domain

A segment where a broadcast reaches every device. Routers and VLANs separate them

Class A

First octet 1 to 126, default mask 255.0.0.0 (/8). About 16.7 million hosts per network, for very large organizations

Class B

First octet 128 to 191, default mask 255.255.0.0 (/16). Up to 65,534 hosts per network, for medium-sized organizations

Class C

First octet 192 to 223, default mask 255.255.255.0 (/24). Up to 254 hosts per network, for small networks

Class D

First octet 224 to 239. Reserved for multicast, with no default mask

Class E

First octet 240 to 255. Reserved for experimental use, with no default mask

RFC1918 private ranges

10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. Never routed on the public internet

APIPA

169.254.0.1 to 169.254.255.254. Self-assigned when no DHCP server answers. Local communication only, no internet

Loopback

127.0.0.1 in IPv4 and ::1 in IPv6. A device uses it to reach itself for testing

Public address

Globally unique and routable on the internet. Anything outside the private and reserved ranges

Private address

Reserved for internal networks and not routed on the internet. Reaches the internet through NAT

IPv6 link-local

fe80::/10. Auto-configured and non-routable, used on the local link

IPv6 unique local

fc00::/7. The IPv6 equivalent of private addresses

IPv6 global unicast

2000::/3. Public, routable IPv6 addresses

IPv6 multicast

ff00::/8

OSI model

Seven-layer framework breaking network communication into layers, from raw transmission up to application services

Layer 1 Physical

Moves raw bits across the medium. Cables, connectors, hubs, and the signals themselves. PDU is bits

Layer 2 Data Link

Node-to-node delivery on a local network using MAC addresses, with framing and error detection. PDU is frames

Layer 3 Network

Logical IP addressing and routing packets between separate networks. PDU is packets

Layer 4 Transport

End-to-end delivery with segmentation, flow control, and error recovery. TCP and UDP use port numbers. PDU is segments or datagrams

Layer 5 Session

Sets up, maintains, and tears down sessions between systems, and controls the dialog

Layer 6 Presentation

Translates data formats, encrypts and decrypts, and compresses so the receiving application can read the data

Layer 7 Application

The layer user applications reach the network through. HTTP, DNS, SMTP, and similar services

MAC sublayer

Lower half of Layer 2. Controls access to the physical medium and handles hardware addressing

LLC sublayer

Upper half of Layer 2. Lets multiple upper-layer protocols share one link and provides flow and error control

Mnemonic top to bottom

All People Seem To Need Dominos Pizza

Mnemonic bottom to top

Please Do Not Throw Sausage Pizza Away

Encapsulation

Adding headers as data moves down the stack from the sender, plus a trailer at Layer 2

Decapsulation

Removing headers as data moves up the stack at the receiver

Devices at Layer 1

Hubs, repeaters, cables, and connectors

Devices at Layer 2

Switches, bridges, NICs, and access points

Devices at Layer 3

Routers and Layer 3 switches

MAC address

Used at Layer 2 for local delivery

IP address

Used at Layer 3 for routing between networks

Port number

Used at Layer 4 to identify the service receiving the data

Subnetting

Dividing a larger network into smaller networks by borrowing bits from the host portion for the network portion