# Subnetting

(a) A network starting with IP address 192.168.0.1 has 20 bits for the host portion. What will be the network address, the broadcast address, the subnet mask and the last address in the network?

b) Using a /30 CIDR subnetting scheme results in a network with only two host addresses, a network address and a broadcast address. What use is a network with only two host addresses?

Hello Kingsly

Hmm, this looks suspiciously like a homework question Here goes…

For convenience, we’ll express the first two octets of the IP address in decimal format and the next two in binary, since most of the calculations will occur on those two octets. So, in that form, 192.168.0.1 looks like this: `192.168.00000000.00000001`.

We are told that the address has 20 bits for the host portion which means a subnet mask with 20 ones and 12 zeros. In binary, that is `11111111.11111111.11110000.00000000`. In dot decimal, that is 255.255.240.0. In our hybrid notation, we’ll use the following: `255.255.11110000.00000000`.

We can find the separation of the parts of the address that belong to the host portion and those that belong to the network portion using the subnet mask. Wherever there is a 0, that digit is in the host portion. So the separation occurs here:

`255.255.1111|0000.00000000`
`192.168.0000|0000.00000001`

Now to find the Network address, make all of the host portion zeros. So the network address is:
`192.168.0000|0000.00000000` or `192.168.0.0`

`192.168.0000|1111.11111111` or `192.168.15.255`

`192.168.0000|0000.00000001` or `192.168.0.1`

`192.168.0000|1111.11111110` or `192.168.15.254`

Concerning the second question, a /30 subnet will give you only two usable host addresses. What kind of network would need only two addresses? A point to point network. Such a network can be used to connect two routers together, since no other devices would exist on such a network.

I hope that this is an opportunity for you to learn using these answers and not just to get your homework done Both are important of course, but learning is what this site is all about!

I hope this has been helpful!

Laz

Many thanks for the other time

What books to i have to read or videos to understand networking in depth and network security? Most especially teach others same

Hi Kingsley,

If you are new to networking, it’s best to start here:

Cisco CCNA R&S (it consists of ICND1 and ICND2) covers the basics of networking, explained on Cisco routers and switches.

To learn network security, you first have to understand how the different networking protocols work. Once you understand a protocol, you can learn how to secure it.

Once you understand the basics, you can learn more about VPNs / Firewall, for example in the Cisco ASA course: