Post

7.10 Subnetting, Routing, PGP

Posted
By So Hee Park 4 min read

CC by Aru

💡 Subnetting

  • Subnetting = dividing a class network into smaller subnetworks.

☑️ Purpose

  • Avoid wasting IP addresses.
  • Organize companies into smaller groups/floors/departments.

✅ Examples of IP Capacity

✅ Class B (/16)

☑️ Number of Companies

  • 2^16 = 65536

☑️ Hosts per Company

  • 2^16 - 2 = 65534

☑️ Why Minus 2?

  • One address = network identifier.
  • One address = broadcast address.

✅ Class C (/24)

☑️ Number of Companies

  • 2^24 = 16,777,216

☑️ Hosts per Company

  • 2^8 - 2 = 254

✅ Why Subnetting Exists

  • Large networks waste many IPs.
  • Companies divide networks into smaller subnets.
  • Subnets can be rented/shared.

✅ How to Subnet

☑️ Step 1: Decide Number of Subnets

✔️ Example

  • 8 floors → 8 subnets

☑️ Step 2: Find Number of Subnet Bits (a)

✔️ Formula

  • 2^a ≥ number of subnets

☑️ Examples

✔️ 8 Subnets

  • 2^3 = 8
  • a = 3

✔️ 9 Subnets

  • 2^4 = 16
  • a = 4

✅ Step 3: Count Host Bits (X)

☑️ Example

  • /16
  • Host bits = 16 Xs

✅ Step 4: Calculate Remaining Host Bits (b)

✔️ Formula

  • b = x - a

☑️ Example

  • b = 16 - 3
  • b = 13

☑️ Result

  • Original /16 network divided into:

    • 8 subnets
    • each subnet has 2^13 - 2 hosts

✅ Step 5: Transform Type Address

  • Some X bits become S bits.
  • S bits identify subnet.

✅ Example 2

☑️ Divide 192.168.34.0/24 into 2 subnets

✅ Step 1: Find a

✔️ Formula

  • 2^a ≥ 2

☑️ Result

  • a = 1
  • 1 subnet bit (S)

✅ Step 2: Original Type Address

☑️ Original

  • 192.168.34.xxxxxxxx

☑️ Host Bits

  • 8 host bits.

✅ Step 3: Calculate b

✔️ Formula

  • b = 8 - 1

☑️ Result

  • b = 7

✅ Step 4: Transform Type Address

☑️ New Address

  • 192.168.34.Sxxxxxxx

☑️ New Prefix

  • /25

✅ Subnet 0

☑️ Subnet Bit

  • S = 0

☑️ Type Address

  • 192.168.34.0xxxxxxx/25

✅ Network Address

☑️ Result

  • 192.168.34.0/25

✅ First Host

☑️ Rule

  • All x = 0
  • Last x = 1

☑️ Result

  • 192.168.34.1/25

✅ Last Host

☑️ Rule

  • All x = 1
  • Last bit = 0

☑️ Result

  • 192.168.34.126/25

✅ Broadcast Address

☑️ Rule

  • All x = 1

☑️ Result

  • 192.168.34.127/25

✅ Important Warning

  • In subnetting:

    • last host is not always .254
    • broadcast is not always .255

✅ Subnet 1

☑️ Subnet Bit

  • S = 1

☑️ Type Address

  • 192.168.34.1xxxxxxx

✅ Network Address

☑️ Binary

  • 10000000

☑️ Decimal

  • 192.168.34.128/25

✅ First Host

☑️ Binary

  • 10000001

☑️ Decimal

  • 192.168.34.129/25

✅ Last Host

☑️ Binary

  • 11111110

☑️ Decimal

  • 192.168.34.254/25

✅ Broadcast Address

☑️ Binary

  • 11111111

☑️ Decimal

  • 192.168.34.255/25

💡 IPv4 Routing

☑️ Router

  • Device with programmed routing table.
  • Chooses best path for packets.

✅ Routing Table Columns

☑️ Destination

  • Network you want to reach.

☑️ Mask

  • Subnet mask of destination.

☑️ Gateway

  • First hop taken by message.

✅ Gateway Rules

☑️ Gateway = 0.0.0.0

  • Direct wired connection exists.

☑️ Other Gateway

  • Gateway = closest interface to next router.

✅ Router Connections

  • One interface receives data.
  • Another interface sends data to next hop.

☑️ Example

  • Messages from LAN 1 to Router 2:

    • enter through ETH1.

✅ Routers Connected to the Internet

☑️ Characteristics

  • Destination:

    • 0.0.0.0

  • Mask:

    • 0.0.0.0

☑️ Meaning

  • Represents outside world/internet.

✅ Internal Routers

  • Can also contain 0.0.0.0
  • Gateway points to next router.

✅ Logical vs Physical Maps

☑️ Logical Map

  • Includes:

    • devices
    • IP addresses

☑️ Physical Map

  • Includes:

    • devices only
    • no IPs

☑️ Routing Tables

  • Designed using logical maps.

💡 PGP

  • PGP = Pretty Good Privacy.

  • Uses:

    • public key
    • private key
  • Used for encrypted emails.

✅ PGP Process

✅ Step 1: Create Keys

☑️ EducaMadrid

  • Configuración
  • Claves PGP
  • Crear

☑️ Security Level

  • 4096 bits recommended.

☑️ Private Key

  • Protected with strong password/hash.

✅ Step 2: View Keys

  • Keys visible under:

    • Claves PGP

✅ Step 3: Send Public Key

☑️ Process

  • Redactar

  • Enable:

    • Adjuntar mi clave pública
  • Send email.

✅ Step 4: Receive Other Person’s Public Key

  • Destination sends back their public key.

☑️ Then

  • Import their public key.

✅ Step 5: Verify Public Key

☑️ Check In

  • Claves PGP

✅ Step 6: Send Encrypted Emails

☑️ Process

  • Redactar

  • Enable:

    • Cifrar este mensaje
  • Send email.

✅ Receiving Encrypted Emails

☑️ Process

  • Receiver gets encrypted message.
  • Receiver uses own private key to decrypt.

✅ Public vs Private Keys

☑️ Public Key

  • Shared with others.
  • Used to establish secure connection.

☑️ Private Key

  • Secret.
  • Used to decrypt received messages.
DAW bilingual, Computer System
This post is licensed under CC BY 4.0 by the author.