[Day 1] 1. FCN Revision

A default gateway enables communication between devices on different networks.
It is typically a router but can sometimes be a firewall.

The ping command checks if a device (identified by an IP) is active on the network.
It sends echo requests to the device and expects echo replies in response.
Key outputs of the ping command:
- Bytes: Size of the packet.
- Time: Response time (latency) in milliseconds.
- TTL (Time to Live): The lifespan of a packet in terms of hops (network devices).
  - In Windows, TTL starts at 128.
  - In Linux, TTL starts at 64.
Network Latency:
- Communication within the same network has lower latency.
- Communication outside the network (via the default gateway and multiple routers) has higher latency.

hostname: Displays the name of the current device in Linux.
tracert (Windows): Traces the route and displays the names of devices along the path to a specific IP.
traceroute: Use the
```
traceroute
```
command to trace the route to an IP or domain. Example:
```
traceroute <IP_or_Domain>
```
.

Defines how IP addresses are divided into network and host parts.
Example:
```
255.255.255.0
```
indicates the first three octets are for the network, and the last is for the host.

Sender
Receiver
Message
Transmission Medium: The medium carrying data (e.g., air for voice, wires/wireless for Ethernet).
Protocol: A set of rules that govern communication.

What is a Port?
- A logical endpoint for communication at the Transport Layer of the OSI model.
Port Numbers:
- Range: 1–1024 are known as Well-Known Ports.
- Example:
  - Port 80: Used by Nginx and httpd (web servers).
To check open ports:
- Windows: Use the
```
netstat
```
  command.
- Linux: Use
```
ss
```
  or
```
nmap <IP>
```
  commands.

Private IP:
- Used within a private network.
- Not globally unique and cannot access the internet directly.
Public IP:
- Globally unique and allows internet access.
NAT (Network Address Translation):
- Converts private IPs to public IPs and vice versa.
- Example: Dynamic NAT.

Network Layer (Layer 3):
- Uses IP addresses (logical).
- Routers operate at this layer.
Data Link Layer (Layer 2):
- Switches operate here.
- L2 switches: Operate only at Layer 2.
- L3 switches: Operate at Layer 2 and Layer 3.