Subnetting with Binary

• A bit is on or off
• A nibble is four bits 0000
• A byte is eight bits 0000 0000

Powers of 2: 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048

Examples

Start with a /24, eight free h bits, or values from 0 to 256.

0000 0000

100 hosts

A subnet needs 100 hosts, to represent that in binary, we’d need at least 7 bits, or 128.

A /24 would be too large, 256 IPs, remove a bit, and we get a /25.

10.0.0.0/25.

20 more hosts

We already spent the first portion of the address space, from 0 to 127.

0000 0000 is 0.

1000 0000 is 128.

How many bits to represent 20 hosts? at least 5, or \( 2^5 = 32-2 = 30 \)

So we start with 128, and use at least five bits.

1000 1000

References

Cisco - Binary Game