The other day someone asked me to explain subnetting. It had been a while so I dusted off my CCNA books and attempted to answer his questions. So I thought this would be an ideal time to jot down some notes for future reference.
This isnt a tutorial or guide but just some some notes on how to calulate the different subnetting values (subnet number, number of hosts etc etc).
What is the broadcast address of the network 172.30.233.0 255.255.255.128 ?
- 128 – 256 = 128
- What is the highest number you can make by placing multiple 128`s into 0. None so this is 0.
- (0 + 128) – 1 = 127
Answer : Broadcast address is 172.30.233.127
How many subnets and hosts per subnet can you get from the network 172.30.0.0 255.255.255.240 ?
- 172.30 is a class B RFC 1918 address and has a /12 prefix. So 12 bits of this address we can do nothing with.
- The subnet mask is /28 so this mean we can break the address into the following :
- 28 bits – 12 bits = 16 subnet bits
- 28 bits – 32 bits = 4 host bits
- This means that this subnet number will conisist of [12 network ID bits ] [16 subnet bits] [4 host bits]
- With the following power of 2`s in mind we can calculate the hosts and subnets :
65536 32768 16384 8192 4096 2048 1024 512 256 128 64 32 16 8 4 2 1
- Host bits = go along 4 and then an extra 1 (saves adding them up) then minus 2 due to the the broadcast and subnetnet zero bits. This gives us 14
- Subnet bits = go along 16 and then 1 extra gives us 65536 subnets.
Answer : 65536 subnets and 14 hosts per subnet.
Which subnet does host 172.24.102.208 255.255.255.224 belong to?
- 224 – 256 = 32
- Whats the highest number we can get by placing 32`s into 208 = 192
Answer : 172.24.102.192.
What valid host range is the IP address 192.168.126.95/26 a part of?
- 192 = 256 = 64
- Highest number that you can get from placing 64’s into 95 = 64.
- 64 = Subnet number
- 64 + 1 = First host
- (64 + 64) – 1 = Broadcast
- (64 + 64) – 2 = Last host
Answer : 192.168.126.65-126
What valid host range is the IP address 172.16.93.193/20 a part of?
- 240 = 256 -16
- Highest number that you can get from placing 16’s into 93 = 80.
- 80 = Subnet number
- x.x.80.1 = First host
- x.x.80.1 (add 16 to 80 and minus 1), and place .254 into the last octect = Last Host
Answer: 172.16.80.1 through to 172.16.95.254
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