3Com 3C840 manual Subnets of Class C networks, Mask Binary Subnets Hosts/Subnet

B-2APPENDIX B: IP ADDRESSING

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–1Class C Subnet Masks

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

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