3Com 3C840 - Manual (Page 174)

B-2 A PPENDIX B: IP A DDRESSING
be host 28 on subnetwork 63 of that network. The network itself would
be called 128.5.0.0 (Class B network number 5).
Notice that by using subnet masks, you can define a natural hierarchy in
which the addresses themselves indicate how a packet is to be routed.
However, all routing devices on an IP network must be using the same
subnetting scheme.
Also note that a subnet mask for a given network segment is not part of
the address and is not transmitted with every packet. It is simply a value
which is known to all the routing devices adjacent to that segment.
Subnets of Class C networks
Since Class C networks are by far the most common, we will take a closer
look at subnetting in a Class C network. Table B-1 is a listing of all
possible values for the last octet (byte) in a Class C subnet mask.
Table B–1 Class C Subnet Masks
One important thing must be noticed 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
Mask Binary Subnets Hosts/Subnet
128 10000000 0 0
192 11000000 2 62
224 11100000 6 30
240 11110000 14 14
248 11111000 30 6
252 11111100 62 2
254 11111110 126 0
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