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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