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.
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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…