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 = 193.1.1.0/24

Subnet #0: 11000001.00000001.00000001.000 00000 = 193.1.1.0/27

Subnet #1: 11000001.00000001.00000001.001 00000 = 193.1.1.32/27

Subnet #2: 11000001.00000001.00000001.010 00000 = 193.1.1.64/27

Subnet #3: 11000001.00000001.00000001.011 00000 = 193.1.1.96/27

Subnet #4: 11000001.00000001.00000001.100 00000 = 193.1.1.128/27 

Subnet #5: 11000001.00000001.00000001.101 00000 = 193.1.1.160/27

Subnet #6: 11000001.00000001.00000001.110 00000 = 193.1.1.192/27

Subnet #7: 11000001.00000001.00000001.111 00000 = 193.1.1.224/27 

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

193.1.1.0/27 and for network 193.1.1.0/24 are identical 193.1.1.0. 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