Subnet of 172.16.45.14/30

Unanswered Question
Oct 24th, 2007

If a host on a network has the address 172.16.45.14/30, what is the address of the

subnetwork to which this host belongs?

A. 172.16.45.0

B. 172.16.45.4

C. 172.16.45.8

D. 172.16.45.12

E. 172.16.45.18

How to solve such Class B subnet calculation so fast?

I have this problem too.
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kevin.dorrell Wed, 10/24/2007 - 23:55

You don't need to calculate, and you don't need to know whether it is a class A, B, C or what.

A /30 means there are 4 addresses per subnet - one network address, two hosts, and a subnet broadcast address. It is typically what you would use on a point-to-point link.

So the last octet must be a multiple of 4. So look at the "14", and go down to the nearest multiple of 4. So we have:

172.16.45.12 - the subnet

172.16.45.13 - first host

172.16.45.14 - second host

172.16.45.15 - subnet broadcast

Does that make sense?

Kevin Dorrell

Luxembourg

keller.oliver Mon, 10/29/2007 - 07:36

/30 means the first 30 bits of the IP address belong to the network part of the address, i.e. are relevant for routing. Therefore, there are only two bits in the host part of such an address.

With these two bits, you can build 4 addresses, with the two special addresses network (all 0s) and broadcast (all 1s).

In the example, the first 3 octetts don't matter, the only relevant one is octett 4:

1. /30 translates to 255.255.255.252

2. 252 is binary 11111100

which means nnnnnnhh (net, host)

3. 14 is binary 00001110

4. network part: 000011xx

4. network 00001100 = 12 <--- 172.16.45.12

5. broadcast 00001111 = 15

HTH,

Oliver

xs_echoss Mon, 10/29/2007 - 11:04

Use the following chart:

-----128--192--224--240--248--252--254--255

-----128---64---32---16----8----4----2----1

255--127---63---31---15----7----3----1----0

The first line presents the subnet;

the second line is the magic number;

the bottom line can act as wildcard mask.

eight number represent the 8bits in each octet.

For example:

172.16.45.14/30

subnet is 255.255.255.252(cuz it is 2bits left)

subnetwork will be incremented by 4(cuz 4 is the magic number which is below 252)

the wildcard mask will be 0.0.0.3(the number below 4).

Hope it helps a little.

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