Super subnetting chart

addresses

By Rick Vanover

May 19, 2003, 7:00am PDT

Put simply, supernetting a TCP/IP network address is the opposite of subnetting it. Supernetting

is also known as CIDR (classless interdomain routing) as defined by RFCs 1517, 1518, 1519,

and 1520. In IPv4, CIDR is one way of attempting to manage the shortage of TCP/IP addresses

until IPv6 takes over.

Supernetting in itself does not give you more TCP/IP addresses; however, it provides larger

single networks for use. Here's how to implement supernetting on your network or support a

supernetted network that you may have inherited.

How supernetting works

Supernetting acts to bridge the gap between a Class C network that is limited to 254 addresses

and a Class B network that is too large, with over 65,000 addresses. In this way, it's possible to

have a "logical" network that offers the number of hosts that best suits your situation.

Supernetting achieves this by making a single network that has your specified number of hosts

and corresponding supernet (like a subnet mask). A supernetted address will look like any other

TCP/IP address in dotted decimal format (XXX.XXX.XXX.XXX), but it will have a supernetted

subnet mask. This looks like a normal subnet mask, but the last octet is not 0 (however, the

leading octets of the supernet mask are still 255). Supernetted addresses will require a default

gateway that needs to be supernetted as well.

Address ranges, or blocks, are important in supernetting. They allow you to identify the valid

addresses in a tabular format that helps identify boundaries on networks. There are many tables

you can create or find on the Internet to plan your networks when using supernetting. Figure A

shows a supernetting chart using an example configuration that we'll examine in this article.

Figure A

Supernetting Class C addresses

This represents part of the CIDR/supernetting chart to help determine which supernet option to choose.

CIDR Block Supernet Mask # of Networks* # of Hosts**

/17 255.255.128.0 128 32766

/18 255.255.192.0 64 16382

/19 255.255.224.0 32 8190

/20 255.255.240.0 16 4094

/21 255.255.248.0 8 2046

http://www.ietf.org/rfc/rfc1517.txt
http://www.ietf.org/rfc/rfc1518.txt
http://www.ietf.org/rfc/rfc1519.txt
http://www.ietf.org/rfc/rfc1520.txt
/22 255.255.252.0 4 1022

/23 255.255.254.0 2 510

/24 255.255.255.0 1 254

/25 255.255.255.128 Less than 1* 126

/26 255.255.255.192 Less than 1* 62

/27 255.255.255.224 Less than 1* 30

/28 255.255.255.240 Less than 1* 14

/29 255.255.255.248 Less than 1* 6

/30 255.255.255.252 Less than 1* 2

*Number of full Class C networks—256 or more available addresses **Available addresses—network and broadcast addresses excluded

This is a chart of the /17 through the /30 block of Class C supernets. These ranges are scalable,

helping you select how many networks and hosts you would like to use. You may notice that /24

CIDR block looks familiar, as that is really not a supernetted network but a subnetted single

Class C network with a standard 24-bit subnet.

Calculating supernet addresses

Calculating a supernet address is easy if the approach is organized. Using the chart in Figure A,

determine how many hosts you want to have available on your network and reference that

against the # of Hosts column to select the best match. Then, once you select the appropriate

number of hosts, you can look across the chart and see the corresponding supernet mask. With

that, you will need to determine a valid starting network.

This starting network must meet certain criteria:

 All networks are consecutive from your starting network.

 The third octet of the first network must be an even number (zero is valid for certain

situations).

 When combining eight networks (like the example below), the third octet of the network

number must be evenly divisible by eight.

 Create a table listing the available networks(s), addresses, supernet mask(s), default

gateway(s), and other networking objects to outline the network.

Usage scenario

In this example, we'll need approximately 1,220 IP addresses for a training lab scenario that

involves 150 people, each of whom requires two servers, five network-attached, multiport serial

devices, and their own laptop. We'll also need extra addresses for a few routers (including one

for Internet access) and addresses for the instructors. This example would be a good candidate