Subnetting Made Easy? The “moving stick” and the “magic number” Jim Blanco Aparicio-Levy Technical Center

Subnetting Made Easy First let’s look at the overall requirement. A class C network consists of 4 octets totaling 32 bits. If we use a Class C network such as , we can only make use of the last octet or 8 bits. There are 256 possible combinations of bits “on” or “off” in one octet.

Subnetting Made Easy 256 addresses would result in a very large collision domain. 256 hosts using the “wire” one at a time would render the LAN unusable. In business environments, host addresses are usually divided into groups or subnets for management and security reasons. In addition the first address is reserved for the subnet address and the last for a broadcast address. So we really have 254 available host addresses.

Subnetting Made Easy We could just divide the addresses in the last octet into more manageable blocks or “subnets”: 256/4 = 64 or 4 subnets each with 64 addresses 256/8 = 32 or 8 subnets each with 32 addresses 256/16 = 16 or 16 subnets each with 16 addresses But this is too simple. We must also keep track of subnet and broadcast addresses.

Subnetting: Class C host address First convert the last octet, represented by the decimal number “0”, into 8 binary “0”s to represent 8 bits.

Subnetting: Class C host address By just utilizing the last bit, we have two possible IP addresses.

Subnetting: Class C host address bit off IP address First, with the bit remaining at “0” or off, the IP address is

Subnetting: Class C host address bit off IP address bit on IP address Second, when the bit is “1” or turned on, the IP address is Thus we have 2 possible IP addresses just utilizing the last bit

Subnetting: Class C host address bit off bit on If we use the two last bits, in the on an off positions, we have four possible IP addresses. We could continue with combinations of 3, 4 and more bits up to 8 which would result in 256 combinations of 1 and 0 or potential IP addresses. Remember “0” is a number.

Subnetting: Class C host address bit off bit on Since we cannot use the first address (subnet), 0 or the last address 255 (broadcast) we have 256-2=254 usable addresses. That’s one big collision domain. We need to divide it up into smaller blocks or “subnets”.

Subnetting: Class C host address Hold on. Thought we had 256 addresses? Or is it 254? Hold on. Thought we had 256 addresses? Or is it 254? There are 256 combinations of 1 and 0. There are 256 combinations of 1 and 0. Possible addresses run from.0 to.255. Possible addresses run from.0 to.255. “0” is a number. “0” is a number yields 256 addresses yields 256 addresses. The first “0” is reserved for the subnet and the last “255” is the broadcast address. The first “0” is reserved for the subnet and the last “255” is the broadcast address.

Subnetting: Class C host address So that’s 256 – 2 or 254 usable addresses in our one big subnet. So that’s 256 – 2 or 254 usable addresses in our one big subnet. Next we need to decide how many subnets will meet our networking requirement. Next we need to decide how many subnets will meet our networking requirement.

Subnetting: Class C subnet address bit off st subnet bit on ed subnet The same rule applies to borrowing bits for subnet addresses. Start at the left side. The first bit can be borrowed and turned on or off resulting in 2 subnets.

Subnetting: Class C subnet address Borrowing two bits yields four combinations of bits on and off, or four different combinations and 4 possible subnets

The moving stick Now let’s put it all together with our “moving stick” method Write the last octet in binary

The moving stick possible host addresses Start on the right. Number to the left to show possible numbers of host addresses.

The moving stick possible number of host addresses possible number of subnets Start on the left. Number to the right to show possible numbers of subnets.

The moving stick possible number of host addresses possible number of subnets Draw the “moving stick.” You could have a combination of 4 subnets with 64 addresses each.

The moving stick possible number of host addresses possible number of subnets Move the “stick” to the right. You could have a combination of 8 subnets with 32 addresses each.

The moving stick possible number of host addresses possible number of subnets Move it again. You could have a combination of 16 subnets with 16 addresses each.

Calculate the subnets Use this IP address Our company requires at least 3 subnets with more than 50 hosts per subnet.

The moving stick possible number of host addresses possible number of subnets Look back to our first example. We borrowed two bits. This fits the requirement of our company – 4 subnets each with up to 64 addresses.

The moving stick possible number of host addresses possible number of subnets We could move the “stick” to the right. But a combination of 8 subnets with 32 addresses each does not meet our company’s requirement.

The moving stick add the “magic number” possible number of host addresses possible number of subnets We move the stick back to the left. 64 is our “magic” number”.

Calculate the subnets SubnetRange Broadcast address Add to the “0” subnet by increments of 64, our magic number. We find our 4 subnet addresses.

Calculate the subnets SubnetRange Broadcast address The first usable address is in our first subnet The last usable address is The broadcast address is through.63 totals 64 addresses, our “magic number”

Calculate the subnets SubnetRange Broadcast address Fill in the remaining columns

Ok, I lied. You still have to figure out that pesky subnet mask.

Just because you graph subnets on a piece of paper doesn’t mean your router orJust because you graph subnets on a piece of paper doesn’t mean your router or PC has any idea what you did. We need a subnet mask to enter into the router CLI or your PC’s local area connection propertiesWe need a subnet mask to enter into the router CLI or your PC’s local area connection properties

Subnet Mask binary numbers Renumber your last 8 bits to show the binary equivalent. Draw your stick to show the two borrowed bits. Your subnet mask is = 192.

Subnet Mask binary numbers If you had borrowed 3 bits. Your subnet mask would be = 224.

Subnet Mask binary numbers Move the stick to borrow 4 bits. Your subnet mask would be = 240.

Problem completed Our company required us to borrow 2 bits so our IP address and subnet mask is: Our company required us to borrow 2 bits so our IP address and subnet mask is: