Subnet

Abstract

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• Logical division of a Computer Network into smaller isolated network for better efficiency, security, and manageability
• You can use the visual subnet calculator to plan out the logical division for IPv4

Reserved IP addresses of a subnet

Every subnet reserves:

• First address as network identifier (also known as network address) used by routers in its routing table to represent the destination network
• Last address as broadcast address

Obtain IP Range of a Subnet

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1. Get private IP address with ipconfig getifaddr <network_interface>, example output: 172.31.22.131
2. Get the subnet mask with ipconfig getoption <network_interface> subnet_mask, example output: 255.255.252.0 (equivalent to /22)
3. The bitwise AND operation (&) between the IP and subnet mask determines the network address (172.31.20.0), while the broadcast address (172.31.23.255) is found by setting all host bits to 1

  10101100.00011111.00010110.10000011 (172.31.22.131)
& 11111111.11111111.11111100.00000000 (255.255.252.0)
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  10101100.00011111.00010100.00000000 -> 172.31.20.0

Setting all host bits (last 10 bits to 1)
  10101100.00011111.00010111.11111111 -> 172.31.23.255

• Network range: 172.31.20.0 – 172.31.23.255
• Broadcast IP: 172.31.23.255
• Usable IPs: 172.31.20.1 – 172.31.23.254
• Likely default gateway: 172.31.20.1

Automate the calculation