Assigning Subnet Addresses - Novell NETWARE 6-DOCUMENTATION Manual

For example, subnet 16 of network 130.57.0.0 with mask 255.255.240.0,
130.57.16.0, can be further divided into 16 sub-subnets with 256 hosts each.
(Actually, this division creates 15 sub-subnets with 254 hosts each because
sub-subnet 130.57.31.0, host 0, and host 255 are not used.)
NOTE:
OSPF and RIP II recognize subnet masks and support variable size
subnets. RIP I does not work when the network is partitioned into variable length
subnets because RIP I assumes that all subnets belonging to the same network
use the same subnet mask.

Assigning Subnet Addresses

HINT:
Because RIP I packets do not carry subnet mask information, the RIP I
routing protocol imposes several restrictions on the use of subnets. If you are using
RIP I, use the same subnet mask for all subnets belonging to the same network.
Using RIP II lifts this restriction.
If you are installing the routing software on a network with subnets, use the
subnet mask already established for the network.
Subnet addresses and host addresses are typically assigned in numeric order,
where both the subnet and host addresses are assigned from the right edge of
their field. By this method, the border between the subnet address and the host
address becomes fixed when the first subnet (subnet address = 1) is assigned.
If the number of hosts on a subnet or the number of subnets required exceeds
the limits of the subnet mask, using this method makes it difficult to adjust the
subnet mask because each host must be renumbered.
To prepare for changes in the size of the subnet mask, RFC 1219 suggests that
subnets be assigned from the left of the subnet address field, and that hosts be
assigned, in numeric order, from the right of the host address field. In this way,
the subnet bits become a mirror image of the host bits. (You must still select
an initial subnet mask and use it for all subnets in the network.) For example,
to apply this method to a Class B IP network with a subnet mask of
255.255.255.0, you assign subnet addresses as follows:
1000 0000 (Decimal 128)
0100 0000 (Decimal 64)
1100 0000 (Decimal 192)
0010 0000 (Decimal 32)
...
0000 0001 (Decimal 1)
0000 0010 (Decimal 2)
0000 0011 (Decimal 3)
0000 0100 (Decimal 4)
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Understanding