cidr

CIDR

An IPv4 CIDR expression is shorthand for a range of IP space. E.g.,

192.168.128.0/20

can be understood as two parts in binary terms: a fixed prefix that extends for 20 bits, and the host part consisting of the remaining variable bits.

------- fixed --------/---variable--
11000000 10101000 1000/0000 00000000
    |        |     |      
   192      168   128

A subnet or VLAN carrying this IP space could have 2^12 or 4096 hosts on it. An all-zero or all-ones host may be reserved, so a few under 4096, actually.

Typically CIDR describes a part of a network such as a subnet. The syntax can also be used with a full 32-bit IP address e.g. 192.168.144.19/20 to indicate the netmask that goes with a particular host on a particular subnet.

cidr-tool

This directory contains the source code for a CIDR calculator called cidr-tool. To build it, run 'make' in this directory. Run cidr-tool -h to see its options.

Operations

Netmask to /N

To convert a netmask to a /N (CIDR length) count the set-bits in the netmask. (The mask must have contiguous set bits followed by contiguous clear bits).

./cidr-tool 255.255.255.240
/28

/N to Netmask

To make a netmask from a /N (CIDR length) set the initial N bits of the netmask.

./cidr-tool /28
255.255.255.240

IP-in-CIDR test

To test whether an IP address is inside a CIDR of length /N, clear the low N bits of the IP address. If the resulting IP and CIDR are equal in binary then the IP is inside the CIDR.

Example

IPv4 routing requires that a host and its gateway are in the same subnet.

GATEWAY=10.20.30.1
ADDRESS=10.20.30.27
NETMASK=255.255.255.224

./cidr-tool $NETMASK $ADDRESS $GATEWAY
Addresses in same network

Expansion

To expand a CIDR to the full set of IP addresses it represents, permute the variable portion exhaustively. A CIDR of length /N has 2^(32-N) permutations.

./cidr-tool 192.168.128.0/20
192.168.128.0
...
192.168.143.255

IP range to CIDR

An IP range such as 0.0.0.0 - 130.255.255.255 may expand to multiple CIDR blocks. To generate the list of blocks, fully expand the IP address list, sort it, and start with the most specific CIDR mask possible that represents the first item. That is the IP itself with length /32. As each item in the list is processed, expand the CIDR range so that it includes the new item unless the resulting expansion would encompass addresses outside of the list. In that case, start a new CIDR range instead.

./cidr-tool 0.0.0.0 130.255.255.255
0.0.0.0/1
128.0.0.0/7
130.0.0.0/8