The MicroTrain Blog

Subnetting: Plain & Simple...Part 2

by Larry Adams on November 30th, -0001

Breaking it Down

Determining the network id and broadcast address can appear to be a daunting task, but following these simple steps will insure you get it right each time.

When given and IP address with a CIDR mask, be sure to translate the mask into the full subnet format and stack the two numbers address as seen below.

      172.168.149.142/22      à    172.168.149.142
                                    255.255.252.0

 Once you have completed this step, the next step is to determine the IP Block size. If you have my subnetting chart, the process requires a quick lookup. If not, just isolate the first octet of the subnet mask that is not 255 when reading from left to right. In the case of the above example, third octet (252) is the one we want.

Next, simply subtract the desired subnet mask octet value (252) from 256.

  256 – 252 = 4  ß 4 is the IP block size in the third octet

Now that we are prepared with the properly formatted subnet mask and block size we can begin.

            IP:         172.168.149.142
            Mask:       255.255.252.0

            NetID:     
            Broadcast: 

1.Drop down any values with a 255 in the subnet mask to the same octet in the network id and broadcast address.

IP:         172.168.149.142
         Mask:       255.255.252.0
                      ↓    ↓ 
         NetID:      172.168.   .
         Broadcast:  172.168.   .

2.There may be one of more octets in the subnet mask that have a 0 value. For these octets, place a 0 in the network id and a 255 in the broadcast address.

IP:         172.168.149.142
      Mask:       255.255.252. 0
                               ↓ 
      NetID:      172.168.   . 0
      Broadcast:  172.168.   .255

3.There may be on octet in the subnet mask that has neither a 0 nor 255 values.

a.To determine the network id, take the value from the IP address (149) and divided by the block size (4).


149 ÷ 4 = 37.25

b.Take the whole number result (37) and multiply it by the block size (4). This is the octet value of the network id.


37 x 4 = 148


IP:         172.168.149.142
Mask:       255.255.252. 0
                           
NetID:      172.168.148. 0
Broadcast:  172.168.   .255


 

c.To determine the broadcast address octet value, add the block size (4) to the network id octet value (148) and subtract 1.


148 + 4 - 1 = 151


IP:         172.168.149.142
Mask:       255.255.252. 0
                           
NetID:      172.168.148. 0
Broadcast:  172.168.151.255

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