🌐 Computer Network Notes: CIDR, Subnetting, Internet Gateway, and More

🧮 CIDR (Classless Inter-Domain Routing)

Definition: CIDR is a method for allocating IP addresses and routing that replaces the older classful addressing system. It allows more efficient use of IP address space.
Key Points:

• Introduced in 1993 to replace Class A, B, and C networks.
• Uses the format: IP_address/prefix_length (e.g., 192.168.1.0/24).
• The prefix length indicates the number of bits used for the network portion of the address.
• Example:
  • 192.168.1.0/24 → 255.255.255.0 (256 addresses)
  • 10.0.0.0/16 → 255.255.0.0 (65,536 addresses)
    Advantages:
• Efficient IP address utilization.
• Reduces the size of routing tables.
• Supports route aggregation (supernetting).
  Disadvantages:
• Slightly more complex to calculate and manage compared to classful addressing.

🧩 Subnetting

Definition: Subnetting divides a single IP network into smaller logical sub-networks (subnets).
Purpose:

• Improves network management, security, and performance.
• Efficient use of available IP addresses.
  Key Concepts:
• A subnet mask identifies the network and host portions of an IP address.
• Example:
  • IP: 192.168.1.100
  • Subnet Mask: 255.255.255.0 → /24
  • Network Address: 192.168.1.0
  • Broadcast Address: 192.168.1.255
    Formula:
• Subnets = 2ⁿ (n = borrowed bits)
• Hosts per subnet = 2ʰ − 2 (h = host bits)
  Advantages:
• Reduces network congestion.
• Better control and security per subnet.
  Disadvantages:
• Complex configuration for large networks.

🌉 Internet Gateway

Definition: An Internet Gateway connects a private network (LAN) to the Internet, acting as a bridge between the two.
Key Functions:

• Provides a path for traffic to reach the Internet.
• Performs Network Address Translation (NAT).
• Enforces firewall and security rules.
  Types:

1. Default Internet Gateway (AWS/Cloud) – Routes traffic from private subnets to the Internet.
2. Hardware/Software Gateways – Routers or firewalls in enterprise setups.
   Advantages:

• Enables Internet connectivity.
• Adds control and security.
  Disadvantages:
• Can become a bottleneck if overloaded.

🔁 NAT (Network Address Translation)

Definition: NAT translates private IP addresses into public IPs before sending packets to the Internet.
Purpose:

• Conserves public IP addresses.
• Adds a security layer by hiding internal IPs.
  Types:

1. Static NAT – One-to-one mapping.
2. Dynamic NAT – Uses a pool of public IPs.
3. PAT (Port Address Translation) – Many private IPs share one public IP via ports.
   Advantages:

• Saves IPs.
• Adds obscurity for internal hosts.
  Disadvantages:
• Can break certain apps (e.g., VoIP).
• Adds processing overhead.

⚙️ DHCP (Dynamic Host Configuration Protocol)

Definition: DHCP automatically assigns IP addresses and other configuration details (DNS, gateway) to devices.
Key Functions:

• Automates IP configuration.
• Uses UDP ports 67 (server) and 68 (client).
  DHCP Process (DORA):

1. Discover – Client broadcasts for IP.
2. Offer – Server offers IP.
3. Request – Client requests the offered IP.
4. Acknowledge – Server confirms.
   Advantages:

• Simplifies IP management.
• Avoids conflicts.
  Disadvantages:
• If server fails, new devices can’t get IPs.
• Not ideal for static devices.

🧭 Summary Table

Summary:

• CIDR enables flexible IP allocation.
• Subnetting divides a network for better management.
• Internet Gateways connect private networks to the Internet.
• NAT conserves public IPs and secures internal systems.
• DHCP automates IP assignment and simplifies configuration.