Subnet Design Facts

Subnet Design Facts

When setting up a network for IP, you will have to make various decisions about the addresses used on the network. Use the following process to identify and assign IP addresses throughout your network.

1. Identify the number of network addresses. Each network segment will require its own network (subnet) address. In addition, each WAN connection must have its own network address (typically assigned by the WAN service provider).
2. Identify the number of hosts for each subnet. You will need one IP address for each device. Be sure to include an IP address for each router interface.
3. Calculate the subnet mask that provides the necessary number of subnet addresses and the number of host addresses per subnet.
4. Identify the valid subnet addresses, and assign them to network segments.
5. Identify valid IP addresses on each subnet (i.e. the host address range).
6. Assign IP addresses to hosts, or plan on using DHCP to dynamically assign IP addresses.

Instead of using formulas and calculations to perform these steps, you can create a table as follows and use it to quickly identify subnet masks, subnet addresses, and host addresses.

Bits in the mask	/25	/26	/27	/28	/29	/30	/31	/32
Magic number	128	64	32	16	8	4	2	1
Decimal mask value	128	192	224	240	248	252	254	255
Hosts per subnet	126	62	30	14	6	2	n/a	n/a
Number of subnets possible (subnet zero)	2	4	8	16	32	64	n/a	n/a

To construct the table, begin by writing the bit-count in the top row. Then compute the remaining rows as follows:

• To get the magic number, start at 128 and split the value in half for each column.
• To get the decimal mask value, add the magic numbers to the left (for example, a /27 mask can be calculated as 128 + 64 + 32 = 224).
• To get the number of hosts per subnet, subtract 2 from the magic number.
• To get the number of subnets, start at 2, then double the number for each column.

As an example of using the table, suppose you are given a subnet address of 199.166.12.32/29. Use the /29 column to perform various subnetting operations. For example:

• The decimal form of the mask is 255.255.255.248.
• There are 6 host addresses on the subnet (33, 34, 35, 36, 37, and 38).
• The next subnet address using this mask is 199.166.12.40 (add the magic number to the first subnet address).
• The broadcast address for the subnet is 199.166.12.39 (subtract 1 from the next subnet address).
• Using this mask, there are a total of 16 possible subnets.

Be aware of the following special cases identified in the table:

• Both /31 and /32 masks cannot be used because there are no host addresses available.
• The table works best for subnetting the last octet. You can still use the table for finding the decimal equivalent of bit-count masks that are less than 24-bits. Just subtract 8 from each number in the first row. For example, a 21-bit mask would use the same column as a 29-bit mask, and would have the same decimal value.
• A /24 subnet has 254 available host addresses (256-2) on a single subnet. The magic number is 256. If you need more hosts than this:
  • Decrease the mask bit count (i.e. /23, /22, /21, and so on).
  • Each time you decrease the bit count, double the magic number, then subtract 2.

 

Credit: Testout 640-802 CCNA Notes