Splitting a Network Into Subnets

A single large network often needs dividing. Subnetting splits one address block into several smaller subnets to separate departments, improve security, and shrink broadcast domains.

You do this by borrowing host bits and turning them into network bits, increasing the prefix length.

To create subnets, you borrow bits from the host portion. Each borrowed bit doubles the number of subnets: borrow 1 bit for 2 subnets, 2 bits for 4, 3 bits for 8.

The formula is 2^(borrowed bits) = number of subnets. Borrowing bits also reduces the hosts available in each subnet.

Take 192.168.1.0/24 and borrow 2 bits, making it /26. That yields 2^2 = 4 subnets, each with 2^6 - 2 = 62 usable hosts.

The new mask is 255.255.255.192. You have turned one 254-host network into four 62-host networks.

The block size (also called the increment) is how far apart the subnets are. It equals 256 minus the interesting octet of the mask.

For /26, the fourth octet of the mask is 192, so the block size is 256 - 192 = 64. Subnets begin every 64 addresses.

Using a block size of 64 on 192.168.1.0/26, the four subnets are:

• 192.168.1.0
• 192.168.1.64
• 192.168.1.128
• 192.168.1.192

Each network address is a multiple of the block size, making them easy to list.

For each subnet, the network address is the multiple of the block size, the broadcast is one below the next subnet, and usable hosts lie between.

For 192.168.1.64/26: network 192.168.1.64, broadcast 192.168.1.127, usable 192.168.1.65 through 192.168.1.126.

The interesting octet is the octet where the mask is neither 255 nor 0 — where the subnetting actually happens. For /26 it is the fourth octet.

For a /18 it would be the third octet (mask 255.255.192.0). Identify the interesting octet first; that is where you apply the block-size math.

Subnetting is not limited to the last octet. Splitting 172.16.0.0/16 into /18 borrows 2 bits in the third octet, giving block size 256 - 192 = 64.

The subnets are 172.16.0.0, 172.16.64.0, 172.16.128.0, and 172.16.192.0 — the same method, just one octet to the left.

Plan by the largest requirement. If one department needs 100 hosts, a /25 (126 usable) fits; a /26 (62) does not.

Choose a prefix that satisfies the biggest subnet, then allocate the resulting blocks. Variable-length subnet masks let different subnets use different sizes for efficiency.

Follow these steps every time:

• Decide how many subnets or hosts you need.
• Borrow enough bits and set the new prefix.
• Compute block size = 256 minus the interesting octet.
• List network, broadcast, and host ranges per subnet.

Practice this until it feels routine.

Subnetting is just bit borrowing plus block-size counting. Once you can find the prefix, block size, and ranges, any question becomes mechanical.

You now hold the complete IPv4 subnetting toolkit: masks, CIDR, host counts, and dividing networks. The next course turns to IPv6.

Split a network.

You learned to split networks into subnets.

• Borrow host bits: subnets = 2^(borrowed bits).
• New prefix lengthens; each subnet holds fewer hosts.
• Block size = 256 - the interesting octet of the mask.
• Subnets fall on multiples of the block size.
• Per subnet: network = the multiple, broadcast = one below the next, hosts between.