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24-port NWay Ethernet Switch User’s Guide
Switch Management and Operating Concepts 81
network was given a Class C IP address that allowed 254 nodes.
Most of these assigned addresses were never used.
CIDR allows IP address space to be assigned in a way that the
total number of available IP addresses in the assigned space
more closely matches the network’s actual needs. This method
has been adapted for use in intranetworks from the Internet.
Under CIDR, the subnet mask notation is reduced to a simplified
shorthand. Instead of spelling out the bits of the subnet mask, it
is simply listed as the number of “1”s (bits) in the network
portion of the address. The subnet mask of the above example
looks like this in binary - 11111111.11100000.00000000.00000000 –
and there are 11 “1”s or 11 bits used to mask the network
address from the node address. So the example written in CIDR
notation becomes:
10.32.0.0/11
This is the reason that non-contiguous bits are not recommended
in subnet masks.
CIDR notation can also be used for classed addresses: Class A =
/8, Class B = /16, and Class C = /24.
# of
Bits Subnet Mask CIDR
Notation # of
Subnets # of
Hosts Total
Hosts
2 255.192.0.0 /10 2 4194302 8388604
3 255.224.0.0 /11 6 2097150 12582900
4 255.240.0.0 /12 14 1048574 14680036
5 255.248.0.0 /13 30 524286 15728580
6 255.252.0.0 /14 62 262142 16252804
7 255.254.0.0 /15 126 131070 16514820
8 255.255.0.0 /16 254 65534 16645636
9 255.255.128.0 /17 510 32766 16710660
10 255.255.192.0 /18 1022 16382 16742404