If you wanted to find out how many of these IP Addresses you can actually use, in other words, assign to hosts, then you simply take 2 IP Addresses from that number (the first and last IP Address), so you're left with 4094 IP Addresses to play with :) There is one thing you must be careful of though ! The value 4096 does not represent the valid, usable IP Addresses. For example, when we use a /20 CIDR Block, which means a Subnet (or Supernet) mask of 255.255.240.0, we can have up to 4096 hosts. This value represents the number of hosts per Supernet. For example, the /24 CIDR Block, 255.255.255.0 Supernet mask is 1 Class C Network, whereas a /20 CIDR Block, 255.255.240.0 Supernet mask is 16 Class C networks. This number shows us how many Class C Networks are combined by using a specific Supernet mask or, if you like, CIDR Block. For example, a /24 CIDR block means a 24 bit Subnet mask, which in its turn translates to 255.255.255.0 :) Simple stuff ! Number of Class C Networks It's also the decimal value the CIDR Block specifies. When you configure the devices that will be attached to the specified network, this is the value you will enter as a Subnet mask. The Supernet Maskīasically, this is your Subnet mask. With such a subnet mask you will have 4 IP Addresses, from which 2 will be reserved (one for the Network address and one for the Broadcast address) and you're left with 2 that you can assign to your hosts (router and firewall). If you have dealt with any ISPs and IP Address assignments, chances are you would have been given your IP Addresses in CIDR format.Ī good example is if you wanted a permanent connection to your ISP and only required 2 IP Addresses, one for your router and one for your Firewall, you would be assigned one /30 CIDR Block. The fact is that those particular CIDR Blocks are valid, regardless of whether applying them to a network means we are Subnetting and not Supernetting. Now you might wonder why we have them in the chart. With CIDR Blocks /25 to /30 we are actually Subnetting and not Supernetting ! When we use a CIDR Block of 24 (24 bit subnet mask) we are not Supernetting ! This is a default subnet mask for a Class C network. These blocks are highlighted in blue and yellow. We would also like you to pay particular attention to the CIDR Block /24, and /25 to /30. Note: In the above paragraph, we called the 14 bits as a subnet mask, when in fact it's a supernet mask, but because when you configure any network device, the field you will need to enter the value is usually named as the 'subnet mask', we decided to name it 'subnet mask' aswell, in order to avoid confusion. For example, /14 means 14 bits assigned to the subnet mask, it is a lot easier telling someone you have a 14 bit subnet mask rather than a subnet mask of 255.252.0.0 :) The CIDR Block simply represents the number of bits used for the subnet mask. Not responsible for any loss resulting from the use of this site.Let's explain the meaning of each column, although you probably already know most of them. Version 3.0 - Version Date: September 20, 2005
CIDR TO SUBNET MASK TABLE LICENSE
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CIDR TO SUBNET MASK TABLE PLUS
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The TCP/IP Guide - IP Subnetting Summary Tables For Class A, Class B and Class C Networks