Written Lab 3.3 Answers

[smithtc1]

On page 169, "Answers to Written Lab 3.3", the fourth answer says the number of hosts for a /18 subnet is 16,384, when the correct answer is 16,382.

[aguilera]

You are absolutely right!

18 Network Bits gives us a mask of 255.255.192.0
if we are using a 18 bits to represent the networks then we only have 14 bits left to represent the hosts in each network.

2^14=16384 That is our block size

However we can not assign a host the Network number or the broadcast number so we have to subtract two from our block size.

16384-2=16382

How many networks will a /18 prefix create?


What are each of the networks that the /18 prefix would create?


What are the broadcasts on each of the networks that a /18 would create?


What networks would you create using a /26 prefix?

[aguilera]

Lets Consider a class B address. Class B's have a default mask of /16.

If we take one of the 16 host bits and use it for networking then our new mask would be /17.

Network Mask 255.255.128.0
Inverse Mask 0.0.127.255
Prefix /17

Ok, so we took one network bit from the host and are now using it to represent the network right? Right. 2 ^ 1 = 2 (We do not subtract 2 when calculating the number of networks. We only subtract 2 when calculating the number of Hosts for each network)

We can create 2 networks with a mask of 255.255.128.0

What are those two networks?

0 and 128 network.

For instance 172.16.0.0/17 and 172.16.128.0/17

How many host can be placed on each of these two networks?

If we were using 17 of the 32 bits for networking then that means we have 15 bits left over for hosts.
2 ^ 15 = 32768
We can not assign every one of these 32,768 IP addresses to a host. Why not? One of those IP addresses will be used to name the network, and another will be used as a broadcast for that network. So that means we have to subtract two IP addresses from the block of 32,768 IP addresses.

That is where we come up with the equation:

2^X-2= Number of usable IP addresses

Network 172.16.0.0 /17
First assignable IP address is 172.16.0.1/17
Last assignable IP address is 172.16.127.254/17
Broadcast IP is 172.16.127.255/17

Network 172.16.128.0/17
First assignable IP address is 172.16.128.1/17
Last assignable IP address is 172.16.255.254/17
Broadcast IP is 172.16.255.255/17

Good luck!