Six Rules

Six Rules

For Subnet (Network), assign 1 from left side.

Left

→

→

→

→

→

→

→

For Host, assign 0 from right side.

←

←

←

←

←

←

←

Right

Rule: First calculate network bits.
Assign network bits from left to right.
Assign host bits rest ot the field(octet).

Rule 1:

How many subnets?

Answer

2^(subnet bits) = Amount of Subnets.
Or
2^(power of borrowed bits) = Amount of Subnets.

Rule 2:

How many hosts per subnet?

Answer

2^(host bits) – 2 = Amount of Hosts per Subnet.

Rule 3:

What are the valid subnets?

Answer

Block Size:Block size can be defined as the number of all
addresses available in that subnet.
Block Size = 256 – Subnet Mask
Suppose we have a subnet mask /26.
                              /26 = 255.255.255.192
Block Size = 256 – 192 = 64.
Start counting from zero in blocks of 64 until reaching the subnet
mask.
So 0, 64, 128, 192. – these are the valid subnets.

Rule 4:

What is the broadcast address?

Answer

Broadcast Address = Next Subnet – 1.

Rule 5:

What is the first valid host address?

Answer

First valid host = Subnet + 1.

Rule 6:

What is the last valid host address?

Answer

Last valid host = Broadcast - 1.

Remember:
Subnet bits start at the left and go to the right without skipping any bits.