# Network Prefix on Octet Boundar and Variable Length Subnet Masking
###### tags: `elearning` `Multimedia Wireless Network`
###### Note Page : [:link:](https://hackmd.io/@YTL0307/HJEb6jWCo)

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## **Network Prefix on Octet Boundary**
- What happens when a network prefix doesn't fall on an octet boundary?
- We saw that when a network prefix falls on an octet boundary, it is very easy to determine the network portion and the host portion of the address without looking at the binary equivalent.
- The first three octets represent the network number and the last octet represents the host address.
- Remember also that the first number in a subnet is reserved.
- In this example, it is easy to see that the IP addresses ending in zero are the reserved subnet numbers, and those ending in numbers other than zero are hosts.
- Here are the addresses of the first subnet and first host in that subnet, shown in binary and decimal format.
- When a single octet comprises both network and host bits, however, it's not so easy to tell the difference between a network number and a host address. You have to look at the binary equivalent.
- Notice here that with a 27-bit prefix length
- The first available subnet number is actually 192.168.3.0, where all bits in the subnet portion are turned off and the first host address in this subnet is 192.168.3.1.
- Nothing looks different yet.
- The second available subnet is 192.168.3.32, where the far-right bit in the subnet portion is turned on.
- The powers of two table on screen shows you how the bits in the last octet now add up to 32.
- The first host address in this subnet, then, is 192.168.3.33.
- Now this address or host address looks very different, and here is where the purpose of the mask really becomes clear.
- If the network prefix does not fall on an octet boundary. it is impossible to determine the actual subnet number without the mask.

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## **Variable Length Subnet Masking Example**
- Using the subnet mask and converting binary to decimal, we can start to build a table of the network numbers available and the host ranges possible within each network.
- The table shows the first two subnets we just looked at, displaying both the dotted decimal equivalents of the full IP addresses and the last octet of the binary numbers.
- The PC 192.168.3.60 wants to communicate with 192.168.3.66.
- Once again, the first step is to determine if the two devices are on the same network or in this case the same subnet.
- We apply the subnet mask to the source IP address and determine that the subnet number is 192.168.3.32
- Now, if we apply the same subnet mask to the destination IP address, we determine that the subnet number is 192.168.3.64.
- Because the two devices are not on the same network, they must use a router to communicate.
- Remember that Acme's manufacturing facility was assigned the network range of 192.168.3.0/24, which they needed to subnet to meet their needs.
- The table shows each subnet number and the range of assignable host numbers in decimal using a 27-bit subnet mask.
- The table also shows the last octet in binary of each subnet as well as its corresponding host address range.
- Remember that there are two reserved addresses in each subnet.
- Now let's assign a few of these subnets and see what the new IP addressing scheme looks like at the ACME manufacturing facility.
- In the graphic on screen, you'll see that we've assigned three subnets, 192.168,3.0, 192.168.3.32, and 192.168.3.64 to the three different networks.
- We've also assigned 3 host numbers from each subnet to the network devices within each subnet.
- Notice that the router has 3 network interfaces in three different subnets, so it must have a unique IP address for each.