Most linux literature I have read on TCP/IP avoids classless networking, but it has actually been in common use on the internet for quite some time. I even have an IP calculator on my palm pilot. There are also graphical tools such as gipsc which is a GNOME IP subnet calculator. There are two commandline tools for *nix, cidr and ipsc. There are numerous programs for Linux and other OS's that do subnet calculation for you. 's helped to show you where the networks ended and the hosts began. With traditional classful networks, just looking at the. ĬIDR can be a bit hard on humans to calculate. It can be described as a further subnet of the traditional Class B (/16) network or as a supernet combining 2 traditional Class C sized networks. For example a /23 gives us a network of 512 addresses. We don't have to divide up the common /24 (aka Class C). You will sometimes here network admins referring to networks as a "slash 24" instead of Class C and the like. Applications that accept this notation can easily calculate the network information from the prefix address. This is a convenient shorthand for describing IPs within their network contexts. Thus a Class C address can be described in prefix notation as 192.168.0.1/24. Using this method, we note the network prefix part of an address by adding a "/" followed by the number of bits used for the network part. your subnets then look like this: 192.168.0.0 - 192.168.0.127ġ92.168.0.0 is network address, 192.168.0.127 is broadcast addressġ92.168.0.128 is network address, 192.168.0.255 is broadcast addressĬIDR also gives us a new form of notation referred to alternately as "cidr notation","prefix notation", or "slash notation". Of course you then lose 4 addresses, 2 in each subnet, for the network and broadcast addresses. This network essentially divides a class C network of 256 total addresses into two subnetworks of 128 addresses each. Instead of using 24 (8*3) bits for our network part, we're using 25 bits - hence the change in the last digit of the mask. So this changes our netmasks and we get masks that look like 255.255.255.128 for example. Here, we subdivide our traditional classĬ expanding the netmask by one bit beyond the octet boundary. It's very easy to see how the network breaks on the third octet.īut it's not necessary to do that. The network part and the remaining 8 bits to represent the hosts: Ī traditional Class C network division looks like this using 24 bits for CIDR, Classless Internet Domain Routing, was invented to solve the related problems of IP address shortage. The breaking of IP space on 8-bit boundaries was a convenience mechanism but it's not necessary to break your subnet and host parts on 8-bit boundaries. They're not inexhaustible, of course, and something had to be done. Furthermore, no one was particularly concerned about the potential inefficiency and wastage of IPs when the supply of IPs seemed inexhaustable. When TCP/IP was created, it was simple for humans to understand the address classes because the broke cleanly on octet boundries (right at the dots in an IP address). You may have already noticed that IP address classes are pretty limited in terms of the size of networks you can allocate.
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