Regarding the all 1s subnet, a router requires that each routing table

entry include the prefix length so that it can determine whether a

broadcast (directed or all subnets) should be sent only to the all 1s sub 

net or to the entire network. For example, when the routing table does

not contain a mask or prefix length for each route, confusion can occur

because the same broadcast address (193.1.1.255) is used for both the

entire network 193.1.1.0/24 and the all 1s subnet 193.1.1.224/27. This

issue is illustrated in Figure 14. 

F I G U R E   1 4 .   I d e n t i f y i n g   a   B ro a d c a s t   t o   t h e   A l l   1 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

Defining Host Addresses for Each Subnet

According to Internet practices, the host number field of an IP address

cannot contain all 0 bits or all 1 bits. The all 0s host number identifies

the base network (or subnetwork) number, while the all 1s host number

represents the broadcast address for the network (or subnetwork). 

In our current example, there are 5 bits in the host number field of each

subnet address. This means that each subnet represents a block of 30

host addresses (25  2 = 30, note that the 2 is subtracted because the 

all 0s and the all 1s host addresses cannot be used). The hosts on each

subnet are numbered 1 through 30. 

In general, to define the address assigned to Host #N of a particular

subnet, the network administrator places the binary representation of N

into the subnet's host number field. For example, to define the address

assigned to Host #15 on Subnet #2, the network administrator simply

places the binary representation of 15 (011112 ) into the 5 bits of Sub 

net #2's host number field. 

U N D E R S TA N D I N G   I P   A D D R E S S I N G

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