Calculating a Supernet
Supernetting is a function of the ISP you are obtaining your IP addresses from. If you require
more than the 254 available addresses than the Class C address offers, your ISP will provide you
with a sequential series of Class C addresses.
The actual IP addresses used will depend on the available addresses the ISP has to issue and
the number of sequential addresses you require.
How they arrive at the sequence of addresses is as follows;
Part I  Determining the Number of Class ‘C’ Addresses Required
Determining a Supernet Subnet Mask is similar to a Network Subnet Mask.
The first piece of information is how many host ID’s does the client require?
1.
Number of host addresses required?
Then they determine how many Class ‘C’ addresses you will require using the following formula.
2.
Divide the entry in Step 1 by 254.
(254 being the number of IP addresses available for a Class ‘C’ License.)
If the result is not a whole number, round up the next whole number.
(No values other than 0 after the decimal point. I.e. 20.8661  21.)
This gives them the number of consecutive Class ‘C’ addresses you will require.
They then need to determine your new subnet mask for all the ranges of Class ‘C addresses you
will be assigned and the starting address for the sequence of Class ‘C’ addresses.
3.
Now to calculate the Subnet Mask
Convert the number in step 2 to an 8 bit binary number

Binary equivalent =
Take the binary value in the previous step, and consider the following;

If the 8 bit binary number has only a single bit set, convert all bits to the left of the bit
to 1’s for the new subnet mask.
(This only applies to networks requiring 2,4,8,16,32,or 64, Class C Addresses)
 00001000, the 4th bit is the only set bit, convert all bits to the left to 1’s
 11111000, is now the new subnet mask.
 Decimal 248

If the 8 bit binary number has multiple bits set, convert all bits to the left of the
leftmost set bit to 1’s and all the bits to the right of the changed bits to 0’s
 000-10110, the 5th bit is the leftmost set bit, convert all leftmost 0’s to 1’s
 111-10110, the convert the 5th bit and all others to the right to 0’s
 111-00000, gives us the new subnet mask.
 Convert this to a decimal number = 224.

The decimal number is the required Subnet Mask Number to replace 255 in the 3rd
Octet.  New Class ‘C’ Supernet Subnet Mask is now 255.255.224.0
Part II  Determining the IP Address starting point

The starting point for a range of addresses is determined from the rightmost set bit in
the subnet mask.
 11100000, gives us an increment value of 32. The rightmost set bit
represents 32 in binary.
 i.e. the following IP address ranges are valid










228.104.0.0  228.104.31.254
228.104.32.0  228.104.63.254
228.104.64.0  228.104.95.254
228.104.96.0  228.104.127.254
228.104.128.0  228.104.159.254
228.104.160.0  228.104.191.254
228.104.192.0  228.104.223.254
228.104.224.0  228.104.255.254
Each one of the above ranges provides us with 32x254 valid Class ‘C’ addresses
represented by one entry in the routing table.
The above example assumes that the ISP has the available 228.104.xxx.0 Class
‘C’ addresses. If so, then they would determine which range of IP addresses
have all the addresses available.
After a range has been determined, an entry would be made in their routing tables indicating one
IP address to represent the range of Class ‘C’ addresses assigned.
In the above example the range of Class ‘C’ addresses were;






228.104.32.0
228.104.33.0
228.104.34.0
228.104.35.0
228.104.36.0
.
.
.
228.104.63.0
(Starting point)
(End point)
These 32 Class ‘C’ networks could be represented by the following single entry in a routing table.

228.104.32.0
255.255.224.0
228.104.32.1
With 228.104.32.1 as the local port connection on the router.
Alan Palmer MCSE, MCT
alan@palmer-associates.com