IP Addressing and Subnetting 257
Step 6
For any boring octets to the right of the interesting octet, record a
value of 255 in the broadcast address (the same number as in Step
5.) One of the four octets should still be empty--the interesting
octet.
Step 7
Examine the interesting octet of the original IP address. Discover
the multiple of the multiplier closest to this number but greater
than the number. Subtract 1 from this multiple. Write down this
value (1 less than the integer multiple of the multiplier) in the
interesting octet of the broadcast address.
For those of you in doubt, examine the examples that follow. In each case, the steps in the
algorithm are shown. The crux of the algorithm is to search for the integer multiple of the
multiplier that is close to but greater than the value in the interesting octet of the IP address.
2
130.4.101.129
Mask (Step 2)
255.255.252.0
Interesting octet (Step 3)
3
Multiplier (Step 4)
256 252 = 4
Broadcast address (boring octets to the left) (Step 5)
130.4.____._____
Broadcast address (boring octets to the right) (Step 6)
130.4._____.255
Broadcast address (104 is the closest multiple of 4 and is greater than 101;
104 1 = 103) (Step 7)
257
Address (Step 1)
199.1.1.5
Mask (Step 2)
255.255.255.224
Interesting octet (Step 3)
4
Multiplier (Step 4)
256 224 = 32
Broadcast address (boring octets to the left) (Step 5)
199.1.1.____
Broadcast address (boring octets to the right) (Step 6)
199.1.1._____
Broadcast address (32 is the closest multiple of 32 and is greater than 4;
32 1 = 31) (Step 7)
199.1.1.31
Address (Step 1)
172.100.201.2
Mask (Step 2)
255.255.254.0
Interesting octet (Step 3)
3
Multiplier (Step 4)
256 254 = 2
ch05.fm Page 257 Monday, March 20, 2000 5:06 PM