Subnetting and IP Calculation

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Calculating subnet size

Imagine boundaries 8-16-24-32. Start from smaller boundary, if subnet is smaller than the boundary, subtract subnet from boundary. Example: 133.33.86.7/18

Get number of subnets in a network: 2bits taken

•  /27 = (27-24) = 23 = 8 subnets
  Get number of hosts in a subnet: 2bits free
• /27 = (32-27) = 25= 32 hosts (block size)
  Get number of usable hosts in a subnet: {subsize} - 2
• 256 hosts - 2 = 254 usable hosts

Get number of IPs in a subnet

1. Count free bits
2. /18 gets = 2^(32-18)=16384
3. Number of IPs 16384 , 11111111.11111111.11000000.00000000

Get subnets first and last address of 133.33.86.7/18

1. Count the block size
2. For /18 next boundary is 24, counting 2^(24-18)=64
3. So the block size is 64 in decimal view in subnet part of the IP.
4. Considering netmask is on 3rd octet and given IP 133.33.86.7, count
5. 133.33.0.0/18 – not our subnet, before
   1. 133.33.64.0/18 – our subnet because
   2. 133.33.64.0 - Base IP
   3. 133.33.64.1 - First usable
   4. 133.33.127.254 – Last usable
   5. 133.33.127.255– Broadcast
6. 133.33.255.255 - Last
7. 133.33.128.0/18 – Next subnet
   GUIDE FOR SUBNETTING

Conversion of Decimal to Binary

Decimal IP octet (e.g. 133.) will be noted as digit.

graph TD
    A[IP Address: 133.33.64.7] --> B{Split into Octets}
    B --> C1[Octet 1: 133]
    B --> C2[Octet 2: 33]
    B --> C3[Octet 3: 64]
    B --> C4[Octet 4: 7]
    C1 --> D1[Convert to Binary: 10000101]
    C2 --> D2[Convert to Binary: 00100001]
    C3 --> D3[Convert to Binary: 01000000]
    C4 --> D4[Convert to Binary: 00000111]
    D1 --> E[Binary IP: 10000101.00100001.01000000.00000111]
    D2 --> E
    D3 --> E
    D4 --> E

Octets contain bits from 128 to 1 (each following is divided by two)

1	2	3	4	5	6	7	8
128	64	32	16	8	4	2	1
To covert decimal IP address into binary, use this table

1. If digit is more or equals to binary position value, then subtract and add 1 in the position, continue the same for remainder.
2. If digit is less then binary position value, then skip.