Ccna Basic Notes [work] Page

CCNA Basic Notes: Complete Review Draft 1. OSI Model (Layer by Layer) Used for understanding network theory and troubleshooting. | Layer | Name | Function | PDU | Key Devices | |-------|------|----------|-----|--------------| | 7 | Application | User interface (HTTP, FTP, SMTP) | Data | Gateway | | 6 | Presentation | Data format (encryption, compression) | Data | Gateway | | 5 | Session | Session setup & teardown | Data | Gateway | | 4 | Transport | Reliable/unreliable delivery (TCP/UDP) | Segment | Firewall | | 3 | Network | Routing & logical addressing (IP) | Packet | Router | | 2 | Data Link | Physical addressing (MAC), error detection | Frame | Switch/Bridge | | 1 | Physical | Bits over media (voltage, pins) | Bits | Hub/Repeater |

Mnemonic: Please Do Not Throw Sausage Pizza Away

2. TCP/IP Model (4-Layer)

Application (Layers 5-7) Transport (Layer 4) – TCP/UDP Internet (Layer 3) – IP, ICMP, ARP Network Access (Layers 1-2) – Ethernet, Wi-Fi Ccna Basic Notes

3. IP Addressing & Subnetting Basics IPv4 Classes (Classful) | Class | Range | Default Mask | Purpose | |-------|-------|--------------|---------| | A | 1-126 | /8 (255.0.0.0) | Large orgs | | B | 128-191 | /16 (255.255.0.0) | Medium orgs | | C | 192-223 | /24 (255.255.255.0) | Small orgs | | D | 224-239 | N/A | Multicast | | E | 240-255 | N/A | Experimental |

Note: 127.0.0.0/8 is loopback (local host testing).

Private IPv4 Addresses (RFC 1918)

10.0.0.0/8 (10.0.0.0 – 10.255.255.255) 172.16.0.0/12 (172.16.0.0 – 172.31.255.255) 192.168.0.0/16 (192.168.0.0 – 192.168.255.255)

Subnetting Shortcuts

/24 → 256 addresses, 254 usable /25 → 128 addresses, 126 usable (mask 255.255.255.128) /26 → 64 addresses, 62 usable /30 → 4 addresses, 2 usable (common for WAN links) CCNA Basic Notes: Complete Review Draft 1

Formula: Usable hosts = 2^(32 – prefix) – 2 Number of subnets = 2^(borrowed bits) 4. TCP vs. UDP | Feature | TCP | UDP | |---------|-----|-----| | Connection | Connection-oriented | Connectionless | | Reliability | Yes (ACKs, retransmission) | No | | Flow Control | Yes (window size) | No | | Ordering | Yes | No | | Overhead | Higher (20 byte header) | Lower (8 byte header) | | Use cases | HTTP, SSH, FTP, Email | DNS, VoIP, streaming | Well-Known Ports:

SSH – 22 (TCP) HTTP – 80 (TCP) HTTPS – 443 (TCP) DNS – 53 (UDP/TCP) DHCP – 67/68 (UDP)