The binary you actually need

Subnetting is easiest when you understand the binary behind it. The good news is that you need only a small part of binary maths, and it becomes automatic with a little practice.

Bits in an octet

An IPv4 address has four octets of 8 bits. Each bit position has a value:

128  64  32  16   8   4   2   1

To convert a number to binary, go left to right and switch on bits that fit:

192 = 128 + 64           = 11000000
168 = 128 + 32 + 8       = 10101000
 10 = 8 + 2              = 00001010
255 = all bits on        = 11111111

Subnet masks in binary

A subnet mask is a run of ones followed by zeros. The number of ones is the prefix length:

/24 = 11111111.11111111.11111111.00000000 = 255.255.255.0
/26 = 11111111.11111111.11111111.11000000 = 255.255.255.192
/28 = 11111111.11111111.11111111.11110000 = 255.255.255.240

Only a few values can appear in a mask octet, so memorise them:

Bits on: 0    1    2    3    4    5    6    7    8
Value:   0  128  192  224  240  248  252  254  255

Network and host bits

The ones in the mask mark network bits. The zeros mark host bits. With h host bits:

• Total addresses in the subnet = 2 to the power of h.
• Usable hosts = 2 to the power of h, minus 2 (network and broadcast addresses).

Powers of two to know by heart:

2^1=2   2^2=4   2^3=8   2^4=16   2^5=32
2^6=64  2^7=128 2^8=256 2^9=512  2^10=1024

So a /26 has 32 minus 26 = 6 host bits, giving 64 addresses and 62 usable hosts. A /30 has 2 host bits, 4 addresses and 2 usable hosts, which is why /30 was traditionally used for point to point links between routers. A /31 is also allowed on point to point links on modern equipment, giving two usable addresses without network or broadcast.

The block size

The most useful shortcut is the block size: 256 minus the interesting mask octet. For a /26, the mask octet is 192, so the block size is 64. Subnets then start at multiples of 64 in that octet: 0, 64, 128 and 192. We use this shortcut constantly in the next lesson.

Practise converting five numbers to binary and calculating hosts for /25 through /30.