The eight subnet numbers for this example are listed in the following
code sample. The underlined portion of each address identifies the
extended network prefix, while the bold digits identify the 3 bits repre 
senting the subnet number field: 
Base Net: 11000001.00000001.00000001 .00000000 =
Subnet #0: 11000001.00000001.00000001.000 00000 =
Subnet #1: 11000001.00000001.00000001.001 00000 =
Subnet #2: 11000001.00000001.00000001.010 00000 =
Subnet #3: 11000001.00000001.00000001.011 00000 =
Subnet #4: 11000001.00000001.00000001.100 00000 = 
Subnet #5: 11000001.00000001.00000001.101 00000 =
Subnet #6: 11000001.00000001.00000001.110 00000 =
Subnet #7: 11000001.00000001.00000001.111 00000 = 
An easy way to verify that the subnets are correct is to ensure that they
are all multiples of the Subnet #1 address. In this example, all subnets
are multiples of 32: 0, 32, 64, 96, and so on. 
The All 0s Subnet and All 1s Subnet
When subnetting was first defined in RFC 950, it prohibited the use of
the all 0s and the all 1s subnets. The reason for this restriction was to
eliminate situations that could potentially confuse a classful router.
Today a router can be both classless and classful at the same time it
could be running RIP 1 (classful protocol) and BGP 4 (Border Gateway
Protocol Version 4 a classless protocol) at the same time. 
With respect to the all 0s subnet, a router requires that each routing
table update include the route/ pair to differentiate
between a route to the all 0s subnet and a route to the entire network.
For example, when using RIP 1which does not supply a mask or prefix
length with each route, the routing advertisements for subnet and for network are identical With 
out somehow knowing the prefix length or mask, a router cannot tell
the difference between a route to the all 0s subnet and the route to the
entire network. This example is illustrated in Figure 13. 
F I G U R E   1 3 .   D i ff e re n t i a t i n g   B e t w e e n   a   R o u t e   t o   t h e   A l l   0 s
S u b n e t   a n d   t h e   E n t i re   N e t w o r k
1 3






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