subnet-calculations.md

Problem - check if two ips are of same subnet

• ip1: 192.168.10.10/23
• ip2: 192.168.11.10/23

Summary of steps

• Find the subnet_ip, then find the num_hosts in that sub network
• using num_hosts in that subnet find out the start_address, last_address
• if the second ip is in that range then they are of same subnet otherwise not

Get ip,subnet_mask in binary

subnet_mask to binary

first 23 bits set to 1

• subnet_mask_in_bin: 11111111.11111111.11111110.00000000
• subnet_mask_in_dec: 255.255.254.0
• num_bits_in_host: 9 (num of 0s in the last part of the mask after 1)
• num_bits_in_network: 7 (num of 1s in the octet that is not all 0s, in this case 3rd octet)

ip1 network, host portion calculation

convert ip1 to binary

• ip1_in_dec: 192.168.10.10
• ip1_in_bin: 11000000.10101000.00001010.00001010

subnet_ip calculation - apply subnet_mask to ip1

if there's a 1 in subnet mask move the bit in ip1 down

octets:             --  1 --.--  2 --.--  3 --.--  4 --
ip1_in_bin:         11000000.10101000.00001010.00001010
subnet_mask_in_bin: 11111111.11111111.11111110.00000000
                    ------------------------------------
subnet_in_bin:      11000000.10101000.00001010.00000000
subnet_in_dec:      192.168.10.0

find total subnets from subnet mask

• basically choose an octet of the subnet mask that is all 0s
• consider only the 3rd octet of the subnet mask as it is the one determining the subnet
• find number of 1's in that octet (num_bits_in_network)
• num_subnets = 2^num_bits_in_network
• num_hosts_per_subnet = 2^num_bits_in_host - 2

for ip1

• from subnet_mask_in_bin 3rd octet is 11111110
• num_ones = 7
• num_subnets = 2^7 = 128
• num_hosts_per_subnet = 2^9 - 2 = 510
• so there are 128 subnets with each subnet having 510 hosts

find ip address range for subnets

• starting address for subnet (network_in_bin only consider the network portion)

  subnet_in_bin:     11000000.10101000.00001010.00000000
  subnet_in_dec:     192.168.10.0
  last_address:     network_in_dec + 510 (`num_hosts_per_subnet`)
                    192.168.10.0 + 510 => 192.168.11.254 
  first_address:     192.168.10.1

• Note:
  • 192.168.10.0 is not a usable ip address as its reserved for subnet address
  • 192.168.11.255 is not a usable ip address as its reserved for subnet address
  • basically if the last portion of ip is 0 or 255 then it can't be used by the host, which is why we subtract 2 in num_hosts_per_subnet calculation

Summary of steps

• We found out that from ip1: 192.168.10.10/23
  • the subnet ip 192.168.10.0 has 510 hosts
    • start_address: 192.168.10.1
    • last_address: 192.168.11.254
• as ip2: 192.168.11.10/23 is inside the range of start_address and last_address we can say that it belongs to the same network

Resources that can help

• https://www.youtube.com/watch?v=s_Ntt6eTn94
• https://www.calculator.net/ip-subnet-calculator.html
• more detailed + shortcut method
  • https://www.youtube.com/watch?v=qQEaAb_p8_E (understand basics)
  • https://www.youtube.com/watch?v=ZxAwQB8TZsM (shortcut to do it in 7 seconds)