1. Module 10 - Subnetting For Fun and Profit
CCNA I
Module 10 - Subnetting For Fun and Profit

2. Why Subnet?
Subnetting allows you to limit broadcast domains to cut traffic.
It also allows you to control access between subnets.

3. How Subnets Work
The subnet mask tells the router(s) how many bits have been “borrowed” to create the subnets.

4. An Example
Think of a class C network. A Class C network might have an address of with a subnet mask of
The first 3 octets are the network part of the address. The last octet is for the hosts.
In this case you have one network that has 256 addresses ( ), 254 of which can be used is the network address is the broadcast address. Neither of these can be used for hosts.

5. Now let’s Subnet
We will “borrow” three bits from the last octet and use them as part of the binary address.
In binary the subnet mask would then look like this:
In decimal it would look like this:

6.  We Borrowed 3 Bits
That left us with 5 bits in the host address. (8-3=5)
25= 32 addresses -2 for the network address and broadcast address = 30 usable addresses in each subnet.

7. How many subnets?
To figure out the number of subnets just look at the number of bits we borrowed.
Take 2 to the power of the number of bits we borrowed. 23 = 8
Then subtract the first and last address because they are the network and broadcast address. 8-2=6
There are 6 usable subnets with 30 host addresses in each.

8. Now let’s plot them: Network Number Network Address Usable host range
Broadcast Address
1-30 1
33-62 2
65-94 3
97-126 4 5 6 7

9. Please Read Module 10! We’re Running Way Behind.
Tests are already active.
There is a fair amount of detail to go over!