Subnetting

Subnetting #

Subnetting is the process of designating some high-order bits from the host part as part of the network prefix and adjusting the subnet mask appropriately. This divides a network into smaller subnets. The following diagram modifies the subnet mask example by moving 2 bits from the host part to the network prefix to form four smaller subnets each one quarter of the previous size.

Binary form Dot-decimal
IP address 11000000.00000000.00000010.10000010 192.0.2.130
Subnet mask 11111111.11111111.11111111.11000000 255.255.255.192
Network prefix 11000000.00000000.00000010.10000000 192.0.2.128
Host part 00000000.00000000.00000000.00000010 0.0.0.2

Source

  • Enables a much more efficient use of IP addresses compared to class blocks .
  • Don’t think about in terms of network IDs and subnet masks but as binary numbers

Short hand #

Calculating hosts #

2^x - 2 = number of hosts possible on a given subnet

Where x is the number of binary digits you have remaining after the subnet mask. In the following example we extend from /24 to /26 creating 4 subnets. That leaves 6 bits remaining for each subnet. That gives us 2^6 = 64 and 64 - 2 = 62.

Making a Subnet #

  1. Start with a single network ID. In this example we’ll convert 192.168.4.0/24 into 4 subnets.
  2. Convert network ID from step 1 to binary: 11000000.10101000.00000100.00000000
  3. How many subnets are you trying to create? In this example we’re trying to create 4 unique subnets.
    • The formula for determining how many subnets you create is 2^y.
  4. Using this formula 2^2 = 4 and therefore we need to extend the subnet down to /26 because y=2. Note: you extend the subnet down by ‘y’ binary digits to create the number of subnets 2^y.
  5. Extend the network ID by y digits and write out all possible subnets in binary.
    • 4 new network IDs with /26 as a subnet mask:
    11000000.10101000.00000100.00000000
11000000.10101000.00000100.01000000
11000000.10101000.00000100.10000000
11000000.10101000.00000100.11000000

     _Note: the extension of the network IDs to the 26th bit._
  6. Convert network IDs back to dotted decimal
    Network ID Host Range Broadcast Address
    192.168.4.0/26 192.168.4.1 - 192.168.4.62 192.168.4.63
    192.168.4.64/26 192.168.4.65 - 192.168.4.126 192.168.4.127
    192.168.4.128/26 192.168.4.129 - 192.168.4.190 192.168.4.191
    192.168.4.192/26 192.168.4.193 - 192.168.4.254 192.168.4.255
    _Note: The first and last address in the subnet are always reserved for the
    network ID and broadcast address._

Backlinks #