How many hosts can be used for this subnet?

Question

Can someone please explain to me the ''EASIEST'' way to learn this... It's a question from a practice CCNA paper...

An ip subnet of 172.16.200.128 with a subnet mask of 255.255.255.128 provides for how many hosts on a network.

What do you mean the 25 bit subnet says it all. How do you work that out?

Answer 1

to find out how many hosts are available you convert the subnet mast to binary.
255.255.255.128 converts to
11111111.11111111.11111111.10000000
(that is supposed to be 7 zeros at the end)
the ones on the left hand side is the networkside. the zeroes on the right are the host side. the formula for computing the number of hosts is 2 ^ H -2. that is 2 to the power of the number of zeroes - two. in this case 2 ^ 7 - 2. 2^7 = 128
so it is 128 -2. which is 126 valid hosts. The CCNA test will allow you to use the standard windows calculator. It can be used to compute 2^7 by selecting the view then scientific. to use it to compute you press 2 then the "x ^ y" key then 7 finally press "=" this will give you the answer that you must then subtract 2 from

You can also use the calculator to convert to binary
type the number you want to convert then slect view and press binary. the number will convert automatically. just remember to change the view back to decimal when completed

Answer 2

The 25 bit subnet tells it all; you have 128 available IP addresses with 1 reserved for network i.d. and 1 reserved for broadcasts; this means that there are 128-2 or 126 availalbe.

Actually this is abreviated as 172.16.200.128/25; the available IP are from .129 to .255; default gateway is usually assigned to the .129 but need not be; .128 is the identifier and .256 is the broadcast.

Your question about "Easiest" way to learn reminds me of one of my calculus students who was having a problem with related rates in first semister calculus. There is no easy way, just burn the midnight oil and learn it. Remember that whenever there is a 1 in the subnet mask it is not subject to alteration within the subnet and actually it all falls into place from that.

Answer 3

http://www.subnet-calculator.com/