You have a network and you have hosts on the network anything with an IP address is a host. Because I am a simple person, I think of it like this; The network number represents the street I live on, and the host portion is used for the numbers on all the houses on my street. A subnet mask of This means we can have computers on this network, because the fourth octet is not being used by the network portion of the address. We know this because of the 0 in the subnet mask We call each of the number sections an octet because we think of them in binary, and there are eight possible bits in each section.
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You have a network and you have hosts on the network anything with an IP address is a host. Because I am a simple person, I think of it like this; The network number represents the street I live on, and the host portion is used for the numbers on all the houses on my street.
A subnet mask of This means we can have computers on this network, because the fourth octet is not being used by the network portion of the address.
We know this because of the 0 in the subnet mask We call each of the number sections an octet because we think of them in binary, and there are eight possible bits in each section. Eight bits is an octet. So our decimal subnet mask Now look at the subnet mask examples again. It becomes important to understand this when you start dividing your network into multiple sub networks. Simply put, subnetting is dividing your network into multiple sub networks.
To go back to my silly example about houses and streets, subnetting gives you multiple streets in your neighborhood. There are two methods for dividing your network into multiple sub networks; One is to simply change your network numbers keeping the same subnet mask. The other is to subnet your network into smaller sub networks. Keeping the same mask: Your network could be divided into two or more networks by changing the network portion of the address such as Example: This is a very common method of dealing with multiple networks.
However, back in the good old days you had to pay for every IP address you used, and if you had 25 computers on your network you probably would not want to pay for addresses! The answer to the problem is Subnetting a network: Subnetting is when you use bits from the host portion of your address as part of your network number. Here is where understanding binary is important. Lets look at a new subnet mask: Which means we are now using some of the binary bits in the fourth octet for our network numbers, and that gives us fewer hosts than our old mask which gave us , but gives us more networks which is why we call it subnetting.
How can we tell how many networks and hosts per network this new subnet mask will give us? The first task is to find out how many bits in the fourth octet are being used? The decimal number is , what is the decimal number as represented in binary? This requires some math - sorry. The formula is: 2n-2, where n is the number of bits being used from the host portion of our subnet mask. Important Note:We subtract 2 networks the first and last subnets from the total unless we have equipment that supports IP Subnet-Zero in which case we use the formula 2n - please see the addendum at the end of this lesson for more details.
Next, we want to know what the network numbers are, and how many hosts we can have on each of the 6 networks? What is the first subnet? The bit that gives us the answer is the 1 closest to the first zero, and in this case it is the 3rd bit from the left. Start adding the value 32 to itself six times to get the six network numbers. Note: A quicker way to find our starting network number is to subtract our mask from
Classless Inter-Domain Routing
Background[ edit ] An IP address is interpreted as composed of two parts: a network-identifying prefix followed by a host identifier within that network. In the previous classful network architecture, IP address allocations were based on the bit boundaries of the four octets of an IP address. An address was considered to be the combination of an 8, 16, or bit network prefix along with a 24, 16, or 8-bit host identifier respectively. Thus, the smallest allocation and routing block contained only addresses—too small for most enterprises, and the next larger block contained addresses—too large to be used efficiently even by large organizations. This led to inefficiencies in address use as well as inefficiencies in routing, because it required a large number of allocated class-C networks with individual route announcements, being geographically dispersed with little opportunity for route aggregation. During the first decade of the Internet after the invention of the Domain Name System DNS it became apparent that the devised system based on the classful network scheme of allocating the IP address space and the routing of IP packets was not scalable.
CIDR Conversion Table
CIDR Blocks Table IPv4 and IPv6