IP Addressing and Subnetting 255
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This example shows a Class C address, with default mask. The broadcast address in that case is a network
broadcast, not a subnet broadcast address.
Binary Algorithm for Deriving the Broadcast Address
The binary algorithm to determine the broadcast address when using no subnetting or basic
subnetting is a little trickier than the similar algorithm for finding the subnet number. The
algorithm shown here is not really what computers use, but it is more instructive about how
broadcast addresses are structured. It starts by repeating the binary algorithm for computing the
subnet number and then adding two short steps. The algorithm is as follows:
Step 1
Write down the IP address in binary.
Step 2
Write down the subnet mask used in this network, in binary,
beneath the binary IP address from Step 1.
Step 3
Record the results of the Boolean AND below the two numbers.
(This is the subnet number.)
Step 4
Copy down the network and subnet bits of the subnet number onto
the next line. This is the beginning of the broadcast address.
Step 5
Fill in the host bit values with all binary 1s. This is the broadcast
address.
Step 6
Convert the result of Step 5 back into decimal, 8 bits at a time.
As usual, a few examples will help:
172.100.2.2
255.255.255.0
172.100.2
172.100.2.255
17.9.44.3
255.255.255.0
17.9.44
17.9.44.255
Address (Step 1)
8.1.4.5
0000 1000 0000 0001 0000 0100 0000 0101
Mask (Step 2)
255.255.0.0
1111 1111 1111 1111 0000 0000 0000 0000
Boolean AND (Step 3)
8.1.0.0
0000 1000 0000 0001 0000 0000 0000 0000
Broadcast address (Steps 4 to 6)
8.1.255.255
0000 1000 0000 0001 1111 1111 1111 1111
Address (Step 1)
130.4.100.1
1000 0010 0000 0100 0110 0100 0000 0001
Mask (Step 2)
255.255.255.0
1111 1111 1111 1111 1111 1111 0000 0000
Boolean AND (Step 3)
130.4.100.0
1000 0010 0000 0100 0110 0100 0000 0000
Broadcast address (Steps 4 to 6)
130.4.100.255
1000 0010 0000 0100 0110 0100 1111 1111
Table 5-18
Subnetting Examples (Continued)
IP Address (Step 1)
Subnet Mask
Network and
Subnet Part (Step 2)
Broadcast Address
(Step 3)
ch05.fm Page 255 Monday, March 20, 2000 5:06 PM