A chart describing CIDR subnets

Admins need to understand some basic networking concepts like CIDR. These are needed when working with AntiSpam & Archiving providers. Here is a simple chart describing the CIDR subnets.

32 = X-network-bits + Y-host-bits
Addresses = 2 ^ Y-host-bits
--------------------------------------------------------------
CIDR        Total number    Network             Description:
Notation:   of addresses:   Mask:
--------------------------------------------------------------
/0          4,294,967,296   0.0.0.0             Every Address
/1          2,147,483,648   128.0.0.0           128 /8 nets
/2          1,073,741,824   192.0.0.0           64 /8 nets
/3          536,870,912     224.0.0.0           32 /8 nets
/4          268,435,456     240.0.0.0           16 /8 nets
/5          134,217,728     248.0.0.0           8 /8 nets
/6          67,108,864      252.0.0.0           4 /8 nets
/7          33,554,432      254.0.0.0           2 /8 nets
/8          16,777,214      255.0.0.0           1 /8 net
--------------------------------------------------------------
/9          8,388,608       255.128.0.0         128 /16 nets
/10         4,194,304       255.192.0.0         64 /16 nets
/11         2,097,152       255.224.0.0         32 /16 nets
/12         1,048,576       255.240.0.0         16 /16 nets
/13         524,288         255.248.0.0         8 /16 nets
/14         262,144         255.252.0.0         4 /16 nets
/15         131.072         255.254.0.0         2 /16 nets
/16         65,536          255.255.0.0         1 /16
--------------------------------------------------------------
/17         32,768          255.255.128.0       128 /24 nets
/18         16,384          255.255.192.0       64 /24 nets
/19         8,192           255.255.224.0       32 /24 nets
/20         4,096           255.255.240.0       16 /24 nets
/21         2,048           255.255.248.0       8 /24 nets
/22         1,024           255.255.252.0       4 /24 nets
/23         512             255.255.254.0       2 /24 nets
/24         256             255.255.255.0       1 /24
--------------------------------------------------------------
/25         128             255.255.255.128     Half of a /24
/26         64              255.255.255.192     Fourth of a /24
/27         32              255.255.255.224     Eighth of a /24
/28         16              255.255.255.240     1/16th of a /24
/29         8               255.255.255.248     5 Usable addresses
/30         4               255.255.255.252     1 Usable address
/31         2               255.255.255.254     Unusable
/32         1               255.255.255.255     Single host
--------------------------------------------------------------

32 = X-network-bits + Y-host-bits

Addresses = 2 ^ Y-host-bits

--------------------------------------------------------------

CIDR Total number Network Description:

Notation: of addresses: Mask:

--------------------------------------------------------------

/0 4,294,967,296 0.0.0.0 Every Address

/1 2,147,483,648 128.0.0.0 128 /8 nets

/2 1,073,741,824 192.0.0.0 64 /8 nets

/3 536,870,912 224.0.0.0 32 /8 nets

/4 268,435,456 240.0.0.0 16 /8 nets

/5 134,217,728 248.0.0.0 8 /8 nets

/6 67,108,864 252.0.0.0 4 /8 nets

/7 33,554,432 254.0.0.0 2 /8 nets

/8 16,777,214 255.0.0.0 1 /8 net

--------------------------------------------------------------

/9 8,388,608 255.128.0.0 128 /16 nets

/10 4,194,304 255.192.0.0 64 /16 nets

/11 2,097,152 255.224.0.0 32 /16 nets

/12 1,048,576 255.240.0.0 16 /16 nets

/13 524,288 255.248.0.0 8 /16 nets

/14 262,144 255.252.0.0 4 /16 nets

/15 131.072 255.254.0.0 2 /16 nets

/16 65,536 255.255.0.0 1 /16

--------------------------------------------------------------

/17 32,768 255.255.128.0 128 /24 nets

/18 16,384 255.255.192.0 64 /24 nets

/19 8,192 255.255.224.0 32 /24 nets

/20 4,096 255.255.240.0 16 /24 nets

/21 2,048 255.255.248.0 8 /24 nets

/22 1,024 255.255.252.0 4 /24 nets

/23 512 255.255.254.0 2 /24 nets

/24 256 255.255.255.0 1 /24

--------------------------------------------------------------

/25 128 255.255.255.128 Half of a /24

/26 64 255.255.255.192 Fourth of a /24

/27 32 255.255.255.224 Eighth of a /24

/28 16 255.255.255.240 1/16th of a /24

/29 8 255.255.255.248 5 Usable addresses

/30 4 255.255.255.252 1 Usable address

/31 2 255.255.255.254 Unusable

/32 1 255.255.255.255 Single host

--------------------------------------------------------------

In networks larger than a /31, one address is used for the network number, another for the broadcast address, and generally another as the default gateway for routing to other networks. A /29 may cover a range of 8 addresses, but only 5 of them can be used as host endpoints. A /30 has only 1 usable address.

Network:   192.168.1.0/30
Gateway:   192.168.1.1
Usable:    192.168.1.2
Broadcast: 192.168.1.3

Network:   192.168.1.0/29
Gateway:   192.168.1.1
Usable:    192.168.1.2-6
Broadcast: 192.168.1.7

Network: 192.168.1.0/30

Gateway: 192.168.1.1

Usable: 192.168.1.2

Broadcast: 192.168.1.3

Network: 192.168.1.0/29

Gateway: 192.168.1.1

Usable: 192.168.1.2-6

Broadcast: 192.168.1.7

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