IP Addressing Basics 243Subnets of Class C networksThe 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 masksTwo important points about the address divisions created by a subnetmask:n RFC 950 requires that the first and last subnet created by a mask arereserved. So, the number of usable subnets is always 2 less than thenumber of divisions created. This makes 128 an unusable netmaskbecause it has no legal subnets!n The first and last host address in each subnet are also reserved (see“Reserved Addresses” below). This means 254 is also an unusablesubnet mask because there are no legal host addresses!Reserved AddressesIn most IP machines, setting all the bits in the host portion of an IPaddress to 1 indicates a broadcast to all nodes on the network. In theClass B network described above, an address of 128.5.255.255 is anetwork broadcast address meaning the packet is destined for all nodeson the entire Class B network. 128.5.63.255 would be a broadcastaddress 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 thehost bits are all set to 0 a broadcast address. For RAS 1500, you configurefor this difference as part of basic setup.On networks with a “high” broadcast address, setting all bits to 0 simplymeans “this host” or “this network” and is usually used only when aMask Binary Subnets Hosts/Subnet128 10000000 0 0192 11000000 2 62224 11100000 6 30240 11110000 14 14248 11111000 30 6252 11111100 62 2254 11111110 126 0
