Subnetting: borrowing bits

Subnetting creates multiple logical networks inside one larger network by borrowing host bits and using them as subnet bits. Bits are borrowed consecutively from the left of the host portion, and the subnet mask records how many. This is what makes networks classless.

The sliding scale

Picture the network/host boundary as a bar you slide right. Slide right (borrow bits) and you get more subnets with fewer hosts each: every borrowed bit doubles the subnet count and roughly halves the host space.

Often that trade is exactly what you want. A small wireless or server VLAN doesn’t need 254 hosts, so carve the space into more, smaller networks.

The two formulas

• Subnets = 2^s, where s is the bits borrowed.
• Hosts per subnet = 2^h - 2, where h is the host bits remaining.

Why minus 2? Two addresses per subnet are reserved and never assigned to devices: the network address (all host bits 0, used in documentation and routing, not on a device) and the directed broadcast (all host bits 1, the last address in the subnet).

So .1 might be the default gateway and .2 the first host, but .0 and the top address are off limits.

New mask from borrowed bits

The place-value chart strikes again. In whichever octet you’re borrowing, the mask octet is the sum of the place values of the borrowed bits:

• Borrow 1 bit: 128, so the mask octet is 128
• Borrow 2 bits: 128 + 64 = 192
• Borrow 3 bits: 128 + 64 + 32 = 224
• Borrow 4: 240. Borrow 5: 248. Borrow 6: 252. Borrow 7: 254. Borrow 8: 255.

Worked examples

Class C, borrow 1: 192.168.52.0/25

The mask goes from 255.255.255.0 to 255.255.255.128. Subnets: 2¹ = 2. Hosts: 7 bits left, so 2⁷ - 2 = 126 per subnet.

Class C, borrow 5: 192.168.52.0/29

Mask 255.255.255.248 (128+64+32+16+8). Subnets: 2⁵ = 32. Hosts: 3 bits left, 2³ - 2 = 6 per subnet.

Class B, borrow 6: 172.16.0.0/22

Mask goes from 255.255.0.0 to 255.255.252.0, the third octet being 128+64+32+16+8+4 = 252. The borrowing happens in the third octet here.

Class A, borrow 8: 10.0.0.0/16

Mask goes from 255.0.0.0 to 255.255.0.0. A whole octet borrowed; a Class A address wearing a Class B-sized mask is classless in action.

The network address itself never changes when you subnet; only the mask does. 192.168.52.0 is the same number at /24 and /25, but at /25 it’s one of two subnets.

Design procedure

1. Start from the network address you were assigned.
2. Count the subnets you need (plan for growth).
3. Work out how many bits to borrow to cover that count.
4. Derive the new mask, binary and decimal.
5. Apply it: list each subnet’s range, network address, and broadcast address.
6. Assign subnet addresses, then host addresses within each.