Tools

Subnetting

1. IP Address

An IP address is a unique numerical identifier assigned to every device on a network that uses the Internet Protocol. It serves to identify the device and its network location. The most common version, IPv4, is a 32-bit number. For example, a web server might have the IP address 172.217.168.14.

2. Dotted Decimal Notation

To make the 32-bit binary IP address more readable for humans, it is converted into dotted decimal notation. This is done by dividing the 32 bits into four 8-bit sections, or octets, and then converting each octet's binary value into a decimal number separated by dots.

  • Example: The binary IP address 10000000.11010000.00000010.10010111 is represented as 128.208.2.151 in dotted decimal notation.

3. Network ID and Host ID

Every IP address is logically divided into two parts: the Network ID and the Host ID. The Network ID identifies the specific network, while the Host ID identifies the device within that network.

  • Example: In the IP address 192.168.1.10, if the Network ID is 192.168.1, then 10 is the Host ID. All devices on the same network, such as 192.168.1.11 and 192.168.1.12, share the same Network ID (192.168.1) but have unique Host IDs.

4. Subnet Mask

A subnet mask is a 32-bit number used to determine which part of an IP address is reserved for Network ID and which is for the Host ID. It consists of 1s for the network portion and 0s for the host portion.

  • Example: For a network, the subnet mask is 255.255.255.128. In binary, this is 11111111.11111111.11111111.10000000.

    Consider a IP address in this network - 192.168.1.16 (11000000.10101000.00000001.00010000)

    • Network ID: The Network ID is the first 25 bits of the IP address. For 192.168.1.10, the Network ID is 192.168.1.0(11000000.10101000.00000001.0).

    • Host ID: The Host ID is the last 7 bits. For 192.168.1.10, the Host ID is 16(0010000).

    Also, possible values of IP addresess belonging to the above network will be in the range : 192.168.1.0 to 192.168.1.127 (11000000.10101000.00000001.00000000 to 11000000.10101000.00000001.01111111)

5. CIDR Notation

CIDR (Classless Inter-Domain Routing) is a modern, flexible method for specifying the network portion of an IP address. It uses a slash (/) followed by a number indicating the number of bits in the network ID. This system replaced the rigid classful addressing system.

  • Example: An IP address of 192.168.1.0 with a subnet mask of 255.255.255.0 can be written in CIDR notation as 192.168.1.0/24. The /24 signifies that the first 24 bits of the address are the network ID. A /16 network would have 16 network bits, and so on.

6. Classes of IP Addresses

  • Class A: For very large networks, with a first octet range of 1-126. Default subnet mask is 255.0.0.0 or /8.
  • Class B: For medium-to-large networks, with a first octet range of 128-191. Default subnet mask is 255.255.0.0 or /16.
  • Class C: For small networks, with a first octet range of 192-223. Default subnet mask is 255.255.255.0 or /24.
  • Class D: Reserved for multicast addressing, with a first octet range of 224-239.
  • Class E: Reserved for experimental use, with a first octet range of 240-255.

7. Subnetting

Subnetting is the process of dividing a large network into smaller, more efficient sub-networks. This is achieved by "borrowing" bits from the host portion of the IP address to create additional network IDs.

Illustration:

A company has a purchased the Class C IP address range 192.168.1.0/24. Now they wish to equally divide the addresses in 2 equal subnets. One subnet will be for the IT Department and the other for the HR Department.

Let's propose a solution to this problem:

1. Complete Network

A typical Class C network has a default subnet mask of /24 (255.255.255.0). In this example, we are given network address 192.168.1.0.

  • Network ID: 192.168.1.0
  • Host ID Range: The last 8 bits of the address represent the host ID, from 192.168.1.0 to 192.168.1.255.

2. Two subnets

To divide the network into two equal parts, we need to "borrow" one bit from the host portion of the address and add it to the network portion. Our original network has a /24 prefix. By borrowing one bit, we extend the network prefix to /25. This new bit allows us to create two subnets because a single bit can have two states: 0 or 1.

  • Original Mask: /24 (255.255.255.0) -> 11111111.11111111.11111111.00000000
  • New Mask: /25 (255.255.255.128) -> 11111111.11111111.11111111.10000000

3. IP Ranges for Each Subnet

The borrowed bit now determines which subnet an IP address belongs to.

Subnet 1: IT Department (Subnet ID = 0)

  • IP Range: 192.168.1.0 to 192.168.1.127
  • Network Address: 192.168.1.0 (all host bits are 0).
  • Broadcast Address: 192.168.1.127 (all host bits are 1).
  • Usable Host Range: 192.168.1.1 to 192.168.1.126 (usable for devices).

Subnet 2: HR Department (Subnet ID = 1)

  • IP Range: 192.168.1.128 to 192.168.1.255
  • Network Address: 192.168.1.128 (all host bits are 0).
  • Broadcast Address: 192.168.1.255 (all host bits are 1).
  • Usable Host Range: 192.168.1.129 to 192.168.1.254 (usable for devices).

Note: The very first IP address in each range is reserved as the network address, and the very last is reserved as the broadcast address. These cannot be assigned to devices.