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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Version 4 (IP, IPv4)                      9  IP Addressing                           9  IP Subnetting: Practical Subnet Design and Address Determination Example

IP Subnetting Step #3: Determining The Custom Subnet Mask 1 2 3 4 5 IP Subnetting Step #5: Determining Host Addresses For Each Subnet

Subnet Formula Calculations With More Than 8 Subnet Bits

When the number of subnet bits is greater than 8, some of the octets are of the form “N divided by an integer”, such as “N/8”. This is an integer division, which means “divide N by 8, keep the integer part and drop the fractional part or remainder”. Other octets are calculated based on the modulo of N, shown as “N%8”. This is the exact opposite: it means, “divide N by 8, drop the integer and keep the remainder”. For example, 33/5 in integer math is 6 (6 with a remainder of 3, drop the remainder, or alternately, 6.6, drop the fraction). 33%5 is 3 (6 with a remainder of 3, drop the 6, keep the remainder).

Let's take as an example our Class B network and suppose that for some strange reason we decided to use 10 bits for the subnet ID instead of 5. In this case, the formula is “x.y.N/4.(N%4)*64”. Subnet #23 in this case would have the address “166.113.23/4.(23%4)*64. The 23/4 becomes just 5 (the fractional.75 is dropped). 23 modulo 4 is 3, which is multiplied by 64 to get 192. So the subnet address is “166.113.5.192”. Subnet #709 would be “116.113.709/4.(709%4)*64, which is 116.113.177.64.

Okay, now for the real fun. If you subnet a Class A address using more than 16 bits for the subnet ID, you are crossing two octet boundaries, and the formulas become very … interesting, involving both integer division and modulo. Suppose we were in charge of Class A address 21.0.0.0 and choose to subnet it. However, we sat down to do this after having had a few stiff ones at the office holiday party, so our judgment is a bit impaired. We decide that it would be a great idea to choose 21 bits for our subnet ID, since we like the number 21. This gives us a couple million subnets.

The formula for subnet addresses in this case, is “x.N/8192.(N/32)%256.(N%32)*8”. Yikes. Well, this is a bit involved—so much so that it might be easier to just take a subnet number and do it in binary, the long way. But let's take an example and see how it works, for, say, subnet #987654. The first octet is of course 21. The second octet is 987654/8192, integer division. This is 120. The third octet is (987654/32)%256. The result of the division is 30864 (we drop the fraction). Then, we take 30864%256, which yields a remainder of 144. The fourth octet is (987654%32)*8. This is 6*8 or 48. So subnet address #987654 is 21.120.144.48.

(Don't drink and drive. Don't drink and subnet either. J)

IP Subnetting Step #3: Determining The Custom Subnet Mask 1 2 3 4 5 IP Subnetting Step #5: Determining Host Addresses For Each Subnet