Subnetting Shortcuts

Class C

200.4.5.0 Borrow 3 bits Subnet Mask = 255.255.255.224

Number of networks created 2³ = 8 Number of useable networks created 2³ = 8 – 2 = 6

Number of host per network 2⁵ = 32 Number of useable host per network 2⁵ = 32 – 2 = 30

The increment for each network is 32

If you need to determine the network number of subnet 6, multiply 6 X 32 = 192. The subnet 6 network address would be 200.4.5.192

In simple terms, multiply the increment by the number of the subnet you wish to find.

Class B – borrowing in the 3^rd octet

150.4.0.0 Borrow 3 bits Subnet Mask = 255.255.224.0

Number of networks created 2³ = 8 Number of useable networks created 2³ = 8 – 2 = 6

Number of host per network 2¹³X 256 = 8192

The increment for each network is 32 in the 3^rd octet (the number of unmasked bits in the 3^rd octet is 5, this is the octet we borrowed from)

If you need to determine the network number of subnet 6, multiply 6 X 32 = 192. The subnet 6 network address would be 150.4.192.0

If you are borrowing in the 3^rd octet, just ignore the 4^th octet to determine your network numbers.

Class B – borrowing all the 3^rd octet

150.4.0.0 Borrow 8 bits Subnet Mask = 255.255.255.0

Number of networks created 2⁸ = 256 Number of useable networks created 2⁸ = 256 – 2 = 254

Number of host per network 2⁰X 256 = 256

The increment for each network is 256 in the 3^rd octet. (The number of unmasked bits in the 3^rd octet is 0, this is the octet we borrowed from. This means that the value of the 3^rd octet increases by one each time.)

If you need to determine the network number of subnet 26, simply insert that number into the 3^rd octet slot. The subnet 26 network address would be 150.4.26.0

If you need to determine the network number of subnet 100, simply insert that number into the 3^rd octet slot. The subnet 100 network address would be 150.4.100.0

Class B – borrowing in the 4^th octet

150.4.0.0 Borrow 11 bits Subnet Mask = 255.255.255.224

Number of networks created 2¹¹ = 2048 (or 2³ X 256 = 2048)

Number of useable networks created 2¹¹ = 2048 – 2 = 2046

Number of host per network 2⁵= 32

The increment for each network is 32 in the 4^th octet. (The number of masked bits in the 4^th octet is 3, the number of unmasked bits in the 4^th octet is 5, this is the last octet we borrowed from. This means that the value of the 4^th octet increases by the increment value.)

If you need to determine the network number of subnet 325, do the following:

Divide the desired subnet by 2 raised to the masked bits in the 4^th octet

325 / 2³or 325 / 8 = 40 remainder 5

The 40 is the value of the 3^rd octet -- 150.4.40.?

To find the value of the 4^th octet multiply the remainder (5) times the increment (32)

32 X 5 = 160 This is the value of the 4^th octet

The subnet 325 network address would be 150.4.40.160

If you need to determine the network number of subnet 40, do the following:

Divide the desired subnet by 2 raised to the masked bits in the 4^th octet

40 / 2³or 40 / 8 = 5 remainder 0

The 5 is the value of the 3^rd octet -- 150.4.5.?

To find the value of the 4^th octet multiply the remainder (0) times the increment (32)

32 X 0 = 0 This is the value of the 4^th octet

The subnet 40 network address would be 150.4.5.0

Another Method

Class B – borrowing in the 4^th octet

150.4.0.0 Borrow 11 bits Subnet Mask = 255.255.255.224

Number of networks created 2¹¹ = 2048 (or 2³ X 256 = 2048)

Number of useable networks created 2¹¹ = 2048 – 2 = 2046

Number of host per network 2⁵= 32

If you need to determine the network number of subnet 325, do the following:

(the X’s represent place holders for the total number of bits borrowed)

X X X X X X X X X X X This represents the 11 bit borrowed

To begin, find the largest number that can be subtracted from the subnet number you wish to find. Place a one in that column and subtract that number from the subnet number. Then do the same with the remainder until the number has been completely subtracted.

The number to begin with = 325

325 – 256 = 69 69 – 64 = 5 5 – 4 = 1 1 – 1 = 0