3Com 3C421600A Reference Manual page 243

Subnets of Class C networks
The following table is a listing of all possible values for the last octet
(byte) in a Class C subnet mask.
Figure 2 Class C subnet masks
Mask Binary
128 10000000
192 11000000
224 11100000
240 11110000
248 11111000
252 11111100
254 11111110
Two important points about the address divisions created by a subnet
mask:
RFC 950 requires that the first and last subnet created by a mask are
reserved. So, the number of usable subnets is always 2 less than the
number of divisions created. This makes 128 an unusable netmask
because it has no legal subnets!
The first and last host address in each subnet are also reserved (see
"Reserved Addresses" below). This means 254 is also an unusable
subnet mask because there are no legal host addresses!
Reserved Addresses
In most IP machines, setting all the bits in the host portion of an IP
address to 1 indicates a broadcast to all nodes on the network. In the
Class B network described above, an address of 128.5.255.255 is a
network broadcast address meaning the packet is destined for all nodes
on the entire Class B network. 128.5.63.255 would be a broadcast
address indicating that the packet is destined for all nodes on subnet 63.
But, one old version of TCP/IP instead considers an address in which the
host bits are all set to 0 a broadcast address. For RAS 1500, you configure
for this difference as part of basic setup.
On networks with a "high" broadcast address, setting all bits to 0 simply
means "this host" or "this network" and is usually used only when a
IP Addressing Basics
Subnets Hosts/Subnet
0 0
2 62
6 30
14 14
30 6
62 2
126 0
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