Subnetting method question

Can someone explain to me this part "We have borrowed from the last octet as the 29th bit is in the last octet. "

How do you figure out which octect is 29 from?

What subnet does 192.168.12.78/29 belong to?

You may wonder where to begin. Well to start with let's find the next boundary of this address.

Our mask is a /29. The next boundary is 32. So 32 - 29 = 3. Now 2^3 = 8 which gives us our block size i.e. 2 to the power of 3 equals 8.

We have borrowed from the last octet as the 29th bit is in the last octet. We start from zero and count up in our block size. Therefore it follows that the subnets are:-

192.168.12.0
192.168.12.8
192.168.12.16
192.168.12.24
192.168.12.32
192.168.12.40
192.168.12.48
192.168.12.56
192.168.12.64
192.168.12.72
192.168.12.80
.............etc

Our address is 192.168.12.78 so it must sit on the 192.168.12.72 subnet.

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Comments

miller811

We need to start with the fundamentals of IP addressing. An IP address is made up of 32 bits, split into 4 octets (oct = 8, yes?). Some bits are reserved for identifying the network and the other bits are left to identify the host.

There are 3 main classes of IP address that we are concerned with.

Class A Range 0 - 127 in the first octet (0 and 127 are reserved)
Class B Range 128 - 191 in the first octet
Class C Range 192 - 223 in the first octet

Below shows you how, for each class, the address is split in terms of network (N) and host (H) portions.

NNNNNNNN . HHHHHHHH . HHHHHHHH . HHHHHHHH Class A
NNNNNNNN . NNNNNNNN . HHHHHHHH . HHHHHHHH Class B
NNNNNNNN . NNNNNNNN . NNNNNNNN . HHHHHHHH Class C

Kaminsky

We have borrowed from the last octet is a really confusing way of phrasing it but it relates back to the theory of what is going on here.

The "Our mask is a /29. The next boundary is 32. So 32 - 29 = 3." threw me as well.

Firstly, look at the class of address from the first number. 192 is a class C which has a default subnet of /24.

The way I have learnt to do it (there are many ways and it is whichever way you are comfortable with is the best way for you) is the 256 - subnet rule. ie 5 bits = 256 - 248(2^5) = 8.

As the IP is a class C this makes the subnet 8 addresses wide.
If it were a class B with /29, it would be 0.8 wide. Class A, 0.0.8 wide.

Now you have that, look at the actual address in the question and work out which subnet boundary the ip address is in as you subnet list shows.

This is just the way I do it but it is quick. I have never done it by boundary way which looks odd as you are working on the hosts bits.

These are good questions to practice over and over in case you get a list of IP addresses and are asked to "select all that are in the same subnet" -> these can also come into play in the "what's wrong with this network" type graphical questions.

SurferdudeHB

Ok got it thank you!