| Mark Scott 2002-10-30, 3:23 pm |
| for those of you taking 216 in the near future, heres an easy way of
subnetting:
1. Look in the question for the network address, and the number of
subnets
2. look at the number of subnets and raise it to the nearest number from:
4, 8, 16, 32, 64 or 128.
3. Knock 2 off this figure
4. using the number in 3 write down the number of 1s as below:
2=2, 6=3, 14=4, 30=5, 62=6, 126=7
5. Pad with zeros until you get 8 digits
6. add along the 1s in the octet to give the number: check - should be
ONLY:
192, 224, 240, 248, 252, or 254
7. Put this number in place of the first 0 octet in the subnet mask:
8. to determine the host numbers convert the subnet to binary and do 2^
the number of zeros:
9. lose 2 from this number This is the number of hosts
10 to do first ID + Increment do 256 - the number in step 6
11 Get the base IP address and append this first ID in
12 The first host is the next address
13 The Broadcast is 1 less than the next subnet
14. The Last host is one less than 13
15. the IDs is the number in 10 added to itself until you reach the
subnet mask
an example will help:
Burger Mac is expanding its presence in the USA, It is opening over 1000
restaurants over the next 3 years and is planning on having a regional
office in each state. Further on in time it wants to open up in Europe and
Africa. The Company envisages that I will not need more than 60 Subnets.
Currently it is running on a 156.19 /16 network. What is the best subnet
mask to use in this case?
1. network address = 156.19.0.0 subnets = 60
2. nearest number = 64
3. 64 - 2 = 62
4. 62 = 6 = 111111
5 11111100
6 128 + 64 + 32 + 16 + 8 + 4 = 252 (fits)
7. 255.255.0.0 = 255.255.252.0
8. 111111111111111111111100000000
00 = 10 = 2^10 = 1024
9 1024 - 2 = 1022 host
10 256 - 252 = 4
11. 156.19.0.0 = 156.19.4.0
12, First host = 156.19.4.1
13. Broadcast = 156.19.7.255
14. Last Host = 156.19.7.254
15. 4,8,12,16,24,28,32......248
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