Nth Subnet Range Calculator

About this interactive

You rarely need every subnet. More often you need one, the 24th or the 300th, and listing every subnet before it is slow and easy to get wrong. The subnets count up in binary in the borrowed bits, starting from all zeros, so the first subnet is number 0, the second is number 1, and subnet n is number n - 1. Write n - 1 in binary across the borrowed bits, fill the host bits with 0 for the network address or with 1 for the broadcast address, and convert back to decimal. Take 172.22.0.0/21, 5 bits borrowed in the third octet. The 21st subnet is number 20, 10100 in five bits. Followed by three host bits of 0, the third octet is 10100000, 160, so the network is 172.22.160.0; with the host bits 1 it is 10100111, 167, and the broadcast is 172.22.167.255. The same shortcut, (n - 1) times the block size, gives the same answer. When the borrowed bits run across an octet boundary, as with 10 bits borrowed on a Class B, the high bits land in the third octet and the low bits at the top of the fourth: subnet 50 of 135.20.0.0/26 is number 49, 0000110001: 00001100 in the third octet is 12, and 01 at the top of the fourth is 64, so it is 135.20.12.64. Type addresses in dotted decimal. Anything you miss shows the answer and how the binary gives it. Seven of the twenty-one questions are drawn each run.

What you'll learn

Calculate the network address, first and last usable addresses and broadcast address of a subnet far down the list, without listing the subnets before it
Use the subnet's number less one, in binary across the borrowed bits, to place a subnet whose boundary falls in any octet

Aligned to

CompTIA Network+

1.7 Given a scenario, use appropriate IPv4 network addressing.

Cisco CCNA

1.6 Configure and verify IPv4 addressing and subnetting

Cisco CCST Networking

2.2 Identify IPv4 addresses and subnet formats

Key terms

Subnet Index: The sequential position number of a subnet within a list of subnets, used to calculate that subnet's addresses without enumerating all preceding subnets.
Network Address: The first address in an IP network range that identifies the network itself and cannot be assigned to a host. It is formed when all host bits are set to zero.
Broadcast Address: The last address in an IP subnet range, formed when all host bits are set to one, used to send a single packet to every device on that network simultaneously.
Usable Host Range: The set of IP addresses within a subnet that can actually be assigned to network devices, spanning from one address above the network address to one address below the broadcast address. For example, in the 192.168.1.0/24 network, the usable host range is 192.168.1.1 through 192.168.1.254.
Binary: The base-2 number system that uses only the digits 0 and 1; the native language of digital computers, which store and process data as electrical on/off states.

Topics

Subnetting Subnet Index Network Address Broadcast Address Interactive Calculation

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