Computer Network Notes

Layer 7: Application Layer
├─ Protocols: HTTP, HTTPS, FTP, SMTP, POP3, DNS
├─ Services: Email, web browsing, file transfer

Layer 6: Presentation Layer
├─ Data encryption, compression, translation
├─ Formats: SSL/TLS encryption

Layer 5: Session Layer
├─ Manages connections between applications
├─ Example: Login sessions, call setup

Layer 4: Transport Layer (TCP/UDP)
├─ Protocols: TCP (reliable), UDP (fast)
├─ Concepts: Ports (80, 443, 22), connections

Layer 3: Network Layer (Routing)
├─ Protocols: IP (IPv4, IPv6)
├─ Devices: Routers
├─ Concepts: IP addressing, routing tables

Layer 2: Data Link Layer
├─ Protocols: Ethernet, Wi-Fi
├─ Devices: Switches, bridges
├─ Concepts: MAC addresses, frames

Layer 1: Physical Layer
├─ Hardware: Cables (fiber, copper), wireless
├─ Concepts: Voltage, wireless frequencies

Characteristics:
- Connection-oriented (establish before sending)
- Guaranteed delivery (no data loss)
- Ordered delivery (arrives in order)
- Slower (extra overhead for reliability)
- Used for: Email, file transfer, web browsing

Example:
Client connects to server
Data sent reliably
Client disconnects

Characteristics:
- Connectionless (no setup)
- No delivery guarantee
- Fast (minimal overhead)
- Used for: Video streaming, online gaming, DNS

Example:
Send packet without connection
Fire and forget

Port: 80 (HTTP), 443 (HTTPS)
Purpose: Transfer web pages
Method: GET (retrieve), POST (submit)
Status codes: 200 (OK), 404 (Not Found), 500 (Error)

Purpose: Translate domain names to IP addresses
Query: What's the IP of google.com?
Response: 142.251.174.102

SMTP: Send mail (port 25, 587)
POP3: Receive mail (port 110)
IMAP: Advanced mail (port 143)

Purpose: Transfer files between computers
Port: 21
Example: Upload website files to server

Notation: 192.168.1.1

Binary: 11000000 . 10101000 . 00000001 . 00000001
         11000000    10101000      00000001    00000001
              64  +       168   +        1  +         1

Address Classes:
Class A: 1.0.0.0 to 126.255.255.255     (Large networks)
Class B: 128.0.0.0 to 191.255.255.255   (Medium networks)
Class C: 192.0.0.0 to 223.255.255.255   (Small networks)

Private IP ranges (can't reach internet):
10.0.0.0 - 10.255.255.255
172.16.0.0 - 172.31.255.255
192.168.0.0 - 192.168.255.255

Network: 192.168.1.0/24
         192.168.1.0 - 192.168.1.255 (256 addresses)
         
Subnet mask: 255.255.255.0 (first 24 bits = network)

Example breakdown:
192.168.1.0   = Network address
192.168.1.1   = Router (gateway)
192.168.1.2-254 = Usable hosts
192.168.1.255 = Broadcast address

Format: 2001:0db8:85a3:0000:0000:8a2e:0370:7334

Advantages over IPv4:
- Larger address space (340 undecillion addresses)
- Better security (IPSec built-in)
- Auto-configuration
- Simplified header

Purpose: Directs packets between networks
Function: Reads IP addresses, forwards packets
Example: Home WiFi router connects your devices to ISP

Purpose: Connects devices within same network
Function: Forwards frames based on MAC addresses
Faster than router (local network only)

Purpose: Connects two different network types
Example: Convert from Ethernet to WiFi

Purpose: Security - blocks unwanted traffic
Function: Inspects packets, applies rules
Rules: Allow/deny based on IP, port, protocol

Distance: Single building or campus
Speed: 100 Mbps - 1 Gbps
Example: Office network connected via Ethernet
Devices: Computers, printers, servers in building

Distance: Multiple cities/countries
Speed: 1 Mbps - 100 Mbps
Example: Internet, branch offices connected
Technology: Leased lines, MPLS, VPN

Standards: 802.11a, 802.11b, 802.11g, 802.11n, 802.11ac
Frequencies: 2.4 GHz, 5 GHz
Range: 30-100 meters
Speed: 11 Mbps to 1+ Gbps
Security: WEP, WPA, WPA2, WPA3

4G LTE: 100 Mbps download
5G: 1+ Gbps download
Used by: Mobile phones, IoT devices

Complete path from Application to Physical:

Application
  (HTTP request to google.com)
           ↓
DNS lookup: google.com → 142.251.174.102
           ↓
Transport Layer: TCP port 443
           ↓
Network Layer: Add IP header with destination 142.251.174.102
           ↓
Data Link: Add MAC address header
           ↓
Physical: Convert to electrical signals
           ↓
Cable/WiFi transmission
           ↓
(Reverse at destination)

DDoS (Distributed Denial of Service):
- Flood server with requests
- Server can't respond to legitimate users

Man-in-the-Middle:
- Intercept communication between two parties
- Read or modify data in transit

Phishing:
- Trick users into revealing credentials
- Fake login pages, emails

HTTPS (SSL/TLS):
- Encrypts data in transit
- Ports 443 (web)
- Protects against eavesdropping

Rules: Allow port 80, 443 (web)
       Block port 23 (telnet)
       Block suspicious IPs

Purpose: Encrypt all traffic through VPN server
Anonymous: Hide your IP address
Secure: Protect on public Wi-Fi

ping google.com              # Test if host is reachable
tracert google.com          # See route packets take
ipconfig /all               # Check IP configuration
netstat -an                 # Show active connections

No Internet:
1. Check router is on
2. Check cables/Wi-Fi connected
3. Run ipconfig to verify IP
4. Ping gateway (usually 192.168.1.1)
5. Ping DNS server (8.8.8.8)

Slow Network:
1. Run speed test (speedtest.net)
2. Check if many devices connected
3. Check Wi-Fi signal strength
4. Restart router
5. Check for malware

DNS Issues:
1. Can't reach by domain (google.com)
2. But can reach by IP (8.8.8.8)
3. Solution: Change DNS to 8.8.8.8 or 1.1.1.1